fix: parse boolean env vars in S3 backend correctly

[vietcgi]

Setting AWS_USE_FIPS_ENDPOINT=false incorrectly enables FIPS endpoints because the code treats any non-empty string as true.

This fixes boolAttrDefaultEnvVarOk to properly parse "true"/"false" (case-insensitive), matching AWS SDK behavior.

Fixes #37601

[crw]

Thanks for this submission! The S3 backend is maintained by the AWS Provider team at HashiCorp, and IBM company. I have notified that team of your PR. Thanks again!

[SaurabhAhuja1983]

@crw Thank you for getting the PR in the queue. Can we expedite this process? It has been a month since it's pending review.

[jar-b]

LGTM 🎉

% TF_ACC=1 go test -count=1 ./...
ok      github.com/hashicorp/terraform/internal/backend/remote-state/s3 202.097s

[jar-b]

Thank you for the contribution, @vietcgi! 👍

I modified the changelog entry to an upgrade note to align with how previous breaking changes to S3 backend behavior have been documented.

[YakDriver]

LGTM 🎉

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.