Prepare for 2.15.0 release

[scpeters]

🎈 2.15.0 Release

Preparation for 2.15.0 release.

Comparison to 2.14.0: ignition-cmake2_2.14.0...ign-cmake2

Needed by gazebosim/gz-plugin#101 and others from gazebo-tooling/release-tools#784

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.