Preparing for release 6.1.1

[lizkenyon]

WHY are these changes introduced?

Preparing for the v6.1.1 patch release.

WHAT is this pull request doing?

• Updates CHANGELOG.md to move the unreleased entry under the v6.1.1 - 2026-03-02 heading
• Bumps the version in src/version.php to 6.1.1

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added a changelog entry, prefixed by the type of change noted above
• I have added/updated tests for this change
• I have updated the documentation for public APIs from the library (if applicable)