Prepare for 8.1.0 Release

[azeey]

🎈 Release

Preparation for 8.1.0 release.

Comparison to 8.0.0: https://github.com/gazebosim/gz-gui/compare/gz-gui8_8.0.0...gz-gui8

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 70.09%. Comparing base (125a5ea) to head (68427a5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## gz-gui8 #612 +/- ## ======================================== Coverage 70.09% 70.09% ======================================== Files 39 39 Lines 5363 5363 ======================================== Hits 3759 3759 Misses 1604 1604 ```

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