Update Changelog for 7.0.0

[scpeters]

🎈 Changelog for Release

Preparation for 7.0.0 release.

Comparison to 6.13.0: ignition-math6_6.13.0...gz-math7

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
• 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.

[codecov]

Codecov Report

Merging #506 (a7cfc2f) into gz-math7 (1bb63e2) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           gz-math7     #506   +/-   ##
=========================================
  Coverage     99.70%   99.70%           
=========================================
  Files            77       77           
  Lines          7007     7007           
=========================================
  Hits           6986     6986           
  Misses           21       21           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.