Update PULL_REQUEST_TEMPLATE.md

Author: lucix-aws

Diff for: .github/PULL_REQUEST_TEMPLATE.md

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+**[IMPORTANT] We [announced](https://aws.amazon.com/blogs/developer/announcing-end-of-support-for-aws-sdk-for-go-v1-on-july-31-2025) the upcoming end-of-support for AWS SDK for Go v1. We are no longer accepting PRs for additive features to the v1 SDK.**
+
 For changes to files under the `/model/` folder, and manual edits to autogenerated code (e.g. `/service/s3/api.go`) please create an Issue instead of a PR for those type of changes.
 
 If there is an existing bug or feature this PR is answers please reference it here.