Prepare for 9.3.0 release

[caguero]

Prepare for 9.3.0 Release

🎈 Release

Preparation for 9.3.0 release.

Comparison to 9.2.0: gz-math9_9.2.0...gz-math9

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.

[scpeters]

Two PRS have been merged to gz-math9 since I approved

releases/9.3.0...gz-math9

In particular I'd like a review of whether #809 / #830 is safe to backport as I marked #809 as "unsure whether safe to backport"