Prepare for 7.3.0 release

[scpeters]

🎈 Release

Preparation for 7.3.0 release.

Comparison to 7.2.0: ignition-msgs7_7.2.0...ign-msgs7

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 ignition-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 #234 (fd634b9) into ign-msgs7 (8768085) will not change coverage.
The diff coverage is n/a.

❗ Current head fd634b9 differs from pull request most recent head c52ec36. Consider uploading reports for the commit c52ec36 to get more accurate results

@@            Coverage Diff             @@
##           ign-msgs7     #234   +/-   ##
==========================================
  Coverage      85.60%   85.60%           
==========================================
  Files              9        9           
  Lines            924      924           
==========================================
  Hits             791      791           
  Misses           133      133           

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