Prepare for 8.7.0 prerelease

[scpeters]

🎈 Prerelease

Preparation for 8.7.0~pre1 release.

Comparison to 8.7.0~pre1: https://github.com/gazebosim/gz-msgs/compare/ignition-msgs8_8.6.0...ign-msgs8

Needed by https://github.com/gazebosim/gz-transport/pull/305

Checklist

• [X] Asked team if this is a good time for a release
• [x] There are no changes to be ported from the previous major version
  • [x] https://github.com/gazebosim/gz-msgs/pull/295
• [X] No PRs targeted at this major version are close to getting in
• [X] 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.

[codecov[bot]]

Codecov Report

Merging #296 (ce0c0fe) into ign-msgs8 (3435e2b) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-msgs8     #296   +/-   ##
==========================================
  Coverage      96.64%   96.64%           
==========================================
  Files              9        9           
  Lines            953      953           
==========================================
  Hits             921      921           
  Misses            32       32           

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[scpeters]

Should we wait for #295 to be merged first?

yes, I just merged it and have updated the changelog in https://github.com/gazebosim/gz-msgs/pull/296/commits/ce0c0feddc8e5a36858a0a22f37af0e20b194dac