Add union merge driver for RELEASENOTES.md to reduce merge conflicts

[mazhelez]

❔What, Why & How

Almost every PR edits the top region of RELEASENOTES.md, and the Deploy.yaml "Calculate Release Notes" step reads everything from the top of the file down to the first ## vX.Y header. Because every PR touches that same region, having several PRs in flight reliably produces merge conflicts in the changelog.

This adds a repo-root .gitattributes that assigns the built-in union merge driver to RELEASENOTES.md:

RELEASENOTES.md merge=union

With union, git keeps the added lines from both sides during a merge/rebase instead of raising a conflict. union is a built-in git driver, so it takes effect automatically on clone/pull — no per-machine git config is required.

Notes / caveats

• union does not guarantee ordering and can occasionally duplicate a line on rebase — harmless for a bullet-list changelog; just glance at the unreleased section before cutting a release.
• It only takes effect during an actual merge/rebase, so existing open PRs benefit once they merge from main.
• The changelog is stored as LF in the index, so the union driver already merges on consistent line endings.

This is intended as a quick, low-risk mitigation. A more robust fragment-based approach (each PR drops a separate file, assembled at release time) can be considered as a follow-up.

Related to issue: #

✅ Checklist

• Add tests (E2E, unit tests)
• Update RELEASENOTES.md
• Update documentation (e.g. for new settings or scenarios)
• Add telemetry

[Copilot]

Pull request overview

Adds Git’s built-in union merge driver for release notes to reduce conflicts between concurrent PRs.

Changes:

• Configures RELEASENOTES.md with merge=union.
• Documents the rationale in .gitattributes.