fix: Create the same number of IPv6 egress only gateway routes as the number of NAT gateways that are enabled/created

[wiseelf]

Description

Fixes issue with routes creation for IPv6 private subnets when you have following setup:

  private_subnets = [
    "10.0.0.0/24", "10.0.1.0/24", "10.0.2.0/24",
    "10.0.3.0/24", "10.0.4.0/24", "10.0.5.0/24",
  ]

  private_subnet_ipv6_prefixes = [
    1, 2, 3,
    4, 5, 6
  ]
  enable_nat_gateway     = true
  single_nat_gateway     = false
  one_nat_gateway_per_az = true
  create_egress_only_igw = true

thanks to @RunCor399

Motivation and Context

• Resolves IPv6 Route in Route Table (rtb-xxx) with destination (::/0) already exists #1058

Breaking Changes

none

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects
• I have executed pre-commit run -a on my pull request

Before the change:

  # aws_route.private_ipv6_egress[0] will be created
  + resource "aws_route" "private_ipv6_egress" {
      + destination_ipv6_cidr_block = "::/0"
      + egress_only_gateway_id      = (known after apply)
      + id                          = (known after apply)
      + instance_id                 = (known after apply)
      + instance_owner_id           = (known after apply)
      + network_interface_id        = (known after apply)
      + origin                      = (known after apply)
      + route_table_id              = (known after apply)
      + state                       = (known after apply)
    }

  # aws_route.private_ipv6_egress[1] will be created
  + resource "aws_route" "private_ipv6_egress" {
      + destination_ipv6_cidr_block = "::/0"
      + egress_only_gateway_id      = (known after apply)
      + id                          = (known after apply)
      + instance_id                 = (known after apply)
      + instance_owner_id           = (known after apply)
      + network_interface_id        = (known after apply)
      + origin                      = (known after apply)
      + route_table_id              = (known after apply)
      + state                       = (known after apply)
    }

  # aws_route.private_ipv6_egress[2] will be created
  + resource "aws_route" "private_ipv6_egress" {
      + destination_ipv6_cidr_block = "::/0"
      + egress_only_gateway_id      = (known after apply)
      + id                          = (known after apply)
      + instance_id                 = (known after apply)
      + instance_owner_id           = (known after apply)
      + network_interface_id        = (known after apply)
      + origin                      = (known after apply)
      + route_table_id              = (known after apply)
      + state                       = (known after apply)
    }

  # aws_route.private_ipv6_egress[3] will be created
  + resource "aws_route" "private_ipv6_egress" {
      + destination_ipv6_cidr_block = "::/0"
      + egress_only_gateway_id      = (known after apply)
      + id                          = (known after apply)
      + instance_id                 = (known after apply)
      + instance_owner_id           = (known after apply)
      + network_interface_id        = (known after apply)
      + origin                      = (known after apply)
      + route_table_id              = (known after apply)
      + state                       = (known after apply)
    }

  # aws_route.private_ipv6_egress[4] will be created
  + resource "aws_route" "private_ipv6_egress" {
      + destination_ipv6_cidr_block = "::/0"
      + egress_only_gateway_id      = (known after apply)
      + id                          = (known after apply)
      + instance_id                 = (known after apply)
      + instance_owner_id           = (known after apply)
      + network_interface_id        = (known after apply)
      + origin                      = (known after apply)
      + route_table_id              = (known after apply)
      + state                       = (known after apply)
    }

  # aws_route.private_ipv6_egress[5] will be created
  + resource "aws_route" "private_ipv6_egress" {
      + destination_ipv6_cidr_block = "::/0"
      + egress_only_gateway_id      = (known after apply)
      + id                          = (known after apply)
      + instance_id                 = (known after apply)
      + instance_owner_id           = (known after apply)
      + network_interface_id        = (known after apply)
      + origin                      = (known after apply)
      + route_table_id              = (known after apply)
      + state                       = (known after apply)
    }

After the change:

  # aws_route.private_ipv6_egress[0] will be created
  + resource "aws_route" "private_ipv6_egress" {
      + destination_ipv6_cidr_block = "::/0"
      + egress_only_gateway_id      = (known after apply)
      + id                          = (known after apply)
      + instance_id                 = (known after apply)
      + instance_owner_id           = (known after apply)
      + network_interface_id        = (known after apply)
      + origin                      = (known after apply)
      + route_table_id              = (known after apply)
      + state                       = (known after apply)
    }

  # aws_route.private_ipv6_egress[1] will be created
  + resource "aws_route" "private_ipv6_egress" {
      + destination_ipv6_cidr_block = "::/0"
      + egress_only_gateway_id      = (known after apply)
      + id                          = (known after apply)
      + instance_id                 = (known after apply)
      + instance_owner_id           = (known after apply)
      + network_interface_id        = (known after apply)
      + origin                      = (known after apply)
      + route_table_id              = (known after apply)
      + state                       = (known after apply)
    }

  # aws_route.private_ipv6_egress[2] will be created
  + resource "aws_route" "private_ipv6_egress" {
      + destination_ipv6_cidr_block = "::/0"
      + egress_only_gateway_id      = (known after apply)
      + id                          = (known after apply)
      + instance_id                 = (known after apply)
      + instance_owner_id           = (known after apply)
      + network_interface_id        = (known after apply)
      + origin                      = (known after apply)
      + route_table_id              = (known after apply)
      + state                       = (known after apply)
    }

[andrewleegoss1978]

I see nothing broken by this and it fixes the necessary issue.

[antonbabenko]

This PR is included in version 5.7.1 🎉

[github-actions]

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