Add union merge driver for RELEASENOTES.md to reduce merge conflicts (#2333)

### ❔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

<!-- No RELEASENOTES.md entry added: this is repository infrastructure,
not a user-facing AL-Go feature. No new settings, code paths, or
telemetry. -->

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>