[Backport] Add backport-aws-6.x entry for aws package

[mrodm]

Proposed commit message

[Backport] Add backport-aws-6.x entry for aws package

Add a new entry in `.backports.yml` for the `aws` package backport branch
`backport-aws-6.x`, based on version 6.21.0 (commit 913b779a20).

This branch is active (`archived: false`) and configured to strip all
other packages (`remove_other_packages: true`), consistent with other
aws backport entries.

Author's Checklist

• base_version matches the package version at the point the branch diverged from main.
• base_commit is the exact commit SHA (or short SHA) on main that the branch is based on.
• maintained_until is set to the date until which this branch will receive backports, or null if indefinite.
• archived: false — branch is active and should receive backports.
• remove_other_packages: true — only the aws package is kept on the branch.

How to test this PR locally

No local testing required. CI will validate the .backports.yml entry (inventory check) and dry-run the backport branch creation. After merge, the backport-aws-6.x branch will be created automatically by CI.

Related issues

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This PR was generated with the assistance of Claude (claude-sonnet-4-6).

[infra-vault-gh-plugin-prod]

💚 Build Succeeded

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• Commit: ad6ec35

cc @mrodm

[mrodm]

New entry to create backport-aws-6.x branch.

The base commit and base version fields should be updated accordingly before merging this PR once the PR #19828 in charge of the major and spec bump is merged. This is needed in case there is some other PR merged in the meantime updating the versioning for the aws package.

@andrewkroh should we set some maintained_until date ? It could be left as null for now and updated later if needed.

[andrewkroh]

Let's set it to 2027-01-16 (one day after the 8.x end-of-maintainence date from https://www.elastic.co/support/eol).

[mrodm]

@andrewkroh Setting remove_other_packages with value true here.

This means that this backport branch will be intended just for the aws package. All the other packages will be removed in this backport branch, and other packages will require new backport branches.