ci: credit contributors in published release notes

[afonsojramos]

Summary

Adds contributor credit to the GitHub release notes that release-please produces.

release-please's changelog is commit-based and carries no author attribution. Switching its changelog-type to github would gain attribution but discard the emoji conventional-commit sections (#3095), so instead the publish-release step now enriches the release body at publish time:

• fetches GitHub's generated release notes for the tag (driven by the existing .github/release.yml categories),
• extracts the unique PR authors (bots filtered: github-actions, renovate, dependabot) and appends them as a ### 🫶 Contributors section,
• appends GitHub's ## New Contributors section verbatim when present (first-time contributors),
• publishes the draft with the enriched body in the same PATCH.

The @mentions also make GitHub render the contributor avatar strip on the release page. CHANGELOG.md is untouched, and a re-run of an already-published release will not duplicate the sections.

Verification

Dry-ran the exact script against the real v7.1.1 release data: body preserved, ### 🫶 Contributors appended with @afonsojramos @setchy (bot filtered out), New Contributors correctly omitted when empty. actionlint and zizmor clean.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud