Conditionally use cmake_path on cmake < 3.20

[mjcarroll]

🦟 Bug fix

Fixes #423

Summary

This will conditionally use cmake_path if it is available, otherwise use multiple CMake functions to get the same behavior. Note that on the forward port, we can drop the alternate path (https://github.com/gazebosim/gz-cmake/issues/350)

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

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:

Comparison is base (ea68ca0) 97.67% compared to head (dd2ef92) 97.67%. Report is 1 commits behind head on gz-msgs10.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## gz-msgs10 #424 +/- ## ========================================== Coverage 97.67% 97.67% ========================================== Files 27 27 Lines 1163 1163 ========================================== Hits 1136 1136 Misses 27 27 ```

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