Changelog: update for 3.0.0

[scpeters]

🎈 Changelog for Release

Preparation for 3.0.0 release. The release is not imminent; this is just updating the changelog, which was not done with the earlier prerelease.

Comparison to 2.15.0: https://github.com/gazebosim/gz-cmake/compare/ignition-cmake2_2.15.0...gz-cmake3

Needed by <PR(s)>

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
• [X] 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.

[j-rivero]

I added a note about the gz-utils package and put the Breaking changes first in the Migration.md 9789b8d.

Thanks so much Steve for generating the changelog!