fix: AMI lookup should only happen when launch template is created

[demolitionmode]

Description

Given that data.aws_ami.eks_default is only used in the aws_launch_template.this resource, which has a conditional var.create_launch_template, the data block has been updated to have the same conditional. This means that var.cluster_version is no longer needed to be passed when var.create_launch_template is set to false (ie. when the available EKS AMIs make no difference to the resources actually being created).

Motivation and Context

Fixes #2385

Breaking Changes

No breaking changes

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

Tested with change made against latest on master branch, and the variables shown in related issue.

Prior to change, omission of cluster_version results in:

│ Error: Invalid template interpolation value
│
│   on .terraform/modules/self_managed_node_group_iam_role/modules/self-managed-node-group/main.tf line 9, in data "aws_ami" "eks_default":
│    9:     values = ["amazon-eks-node-${var.cluster_version}-v*"]
│     ├────────────────
│     │ var.cluster_version is null
│
│ The expression result is null. Cannot include a null value in a string
│ template.

After change, omission of cluster_version is valid

[bryantbiggs]

makes sense since the launch template is the only consumer of the resource - thank you!

[antonbabenko]

This PR is included in version 19.5.1 🎉

[demolitionmode]

Thank you for your quick review! 🙇

[github-actions]

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