ci: let release-please own CHANGELOG.md

The release pull request failed format:check on both Node versions.
release-please rewrites CHANGELOG.md on every release and its output does
not match Prettier's style, so the two would fight on every single release
and no release PR could ever go green.

release-please owns that file; Prettier now ignores it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: mschmicking

.prettierignore

@@ -3,3 +3,7 @@ dist-test/
 node_modules/
 package-lock.json
 test/fixtures/*.pem
+
+# Generated by release-please on every release; its formatting is not
+# negotiable and format:check would fail the release PR forever.
+CHANGELOG.md