auto_minor_version_upgrade defaults to true and we can not self-update CT

[videlov]

We have currently an issue when self-update our CT:

Error: Error applying plan:

1 error(s) occurred:

* aws_db_instance.default: 1 error(s) occurred:

* aws_db_instance.default: Error modifying DB Instance terraform-xxx: InvalidParameterCombination: Cannot upgrade postgres from 9.6.18 to 9.6.11

AWS had automatically upgraded our RDS instance, because default value for auto_minor_version_upgrade in terraform for it is true:
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/db_instance#auto_minor_version_upgrade

A solution may be to set this property to this to false to prevent such an effect in the future, if you want to keep the hardcoded version. Meanwhile can we bump the hardcoded version to 9.6.18 so we fix our issue as well?

PR to review for this change:
#83

Thanks
@videlov

[DanielJonesEB]

Thanks @videlov - we've been hit by this ourselves! The team are either all on billable work or virtually attending the CF Summit.

@andy-paine @crsimmons @irbekrm @will-gant If you get a chance, can you take a look at this as a priority over any bench work please?

[andy-paine]

Tested on a new install and updating our CT deployment and all looks good. It will take a few hours to clear our CI system then a new release will be cut.

Thanks for the contribution @videlov! 🎉

[crsimmons]

I just cut https://github.com/EngineerBetter/control-tower/releases/tag/0.13.4 with this change. Thanks again for the PR @videlov!