Prepare for 7.1.0 Release

[azeey]

🎈 Release

Preparation for 7.1.0 release.

Comparison to 7.0.0: https://github.com/gazebosim/gz-physics/compare/gz-physics7_7.0.0...gz-physics7

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.32%. Comparing base (b312755) to head (421c2b2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## gz-physics7 #607 +/- ## ============================================ Coverage 78.32% 78.32% ============================================ Files 140 140 Lines 8069 8069 ============================================ Hits 6320 6320 Misses 1749 1749 ```

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