Prepare for 12.1.0 release

[nkoenig]

Signed-off-by: Nate Koenig [email protected]

🎈 Release

Preparation for 12.1.0 release.

Comparison to 12.1.0: sdformat12_12.0.0...sdf12

Needed by:

1. Added magnetometer inspector gz-sim#1173
2. Added altimeter sensor inspector gz-sim#1172
3. Support editing air pressure sensor in the GUI gz-sim#1171
4. Support recreating model entities gz-sim#1170

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
• Link to PR updating dependency versions in appropriate repository in ignition-release

Note to maintainers: Remember to use Squash-Merge