docs: Avoid DB proxy diffs for unsupported AZs

[YakDriver]

Description

This PR updates the documentation to address an issue where Terraform perpetually shows a diff and recreates an RDS Proxy when one or more specified subnets are in an Availability Zone (AZ) that does not support DB proxies.

Problem

When creating an RDS Proxy, the proxy is successfully created but only includes subnets from supported AZs. If one of the specified subnets is in an unsupported AZ:

1. The proxy is created with only the subnets in supported AZs (e.g., supported_1 and supported_2 instead of the expected three subnets).
2. Subsequent Terraform plans detect a difference between the desired state (three subnets) and the actual state (two subnets) and attempt to recreate the proxy. This results in perpetual diffs and recreation cycles.

Root Causes

• The AWS API does not return an error when unsupported AZs are included, unless fewer than two supported AZs are provided.
• There is no definitive, up-to-date source of information listing which AZs globally support DB proxies, making it challenging to validate configurations proactively.

Workarounds

Since the problem cannot be fully resolved at this time, this PR provides documentation updates with practical workarounds:

1. Exclude Unsupported AZs: Use the aws_availability_zones data source to dynamically filter out unsupported AZs when defining subnets.
2. Ignore Changes: Use the lifecycle ignore_changes meta-argument to suppress Terraform's diff detection for the vpc_subnet_ids argument.

Limitations

• These workarounds are not foolproof. Excluding unsupported AZs relies on specific knowledge of which AZs to exclude, which is not always available.
• Ignoring changes with ignore_changes prevents Terraform from managing updates to vpc_subnet_ids, which could lead to configuration drift.

Future Improvements

The best long-term solution would be for the AWS API to return an error when unsupported AZs are included, allowing Terraform to fail fast during resource creation. However, this is not currently possible due to API behavior.

Relations

Closes #17781
Closes #23446

References

Output from Acceptance Testing

% make testacc TESTS=TestAccXXX PKG=ec2

...

[github-actions]

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[jar-b]

LGTM 🚀

[github-actions]

This functionality has been released in v5.81.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!