Prepare for 3.12.1 Release

[azeey]

🎈 Release

Part of gazebo-tooling/release-tools#1223

This will be the final released of ign-gui3 as Citadel has reached EOL.

Preparation for 3.12.1 release.

Comparison to 3.12.0: ignition-gui3_3.12.0...ign-gui3

Checklist

• Asked team if this is a good time for a release
• There are no changes to be ported from the previous major version
• No PRs targeted at this major version are close to getting in
• Bumped minor for new features, patch for bug fixes
• Updated changelog
• Updated migration guide (as needed)
• Link to PR updating dependency versions in appropriate repository in gazebo-release (as needed):

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.