Prepare for 5.7.0

[mjcarroll]

🎈 5.7.0 Release

Preparation for 5.7.0 release.

Comparison to <5.6.0>: ignition-msgs5_5.6.0...ign-msgs5

Needed by gazebosim/ros_gz#137

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

Note to maintainers: Remember to use Squash-Merge

[codecov]

Codecov Report

Merging #144 (2661aad) into ign-msgs5 (c0af147) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-msgs5     #144   +/-   ##
==========================================
  Coverage      84.38%   84.38%           
==========================================
  Files              7        7           
  Lines            807      807           
==========================================
  Hits             681      681           
  Misses           126      126           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c0af147...2661aad. Read the comment docs.