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 (gazebosim/gz-cmake#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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

@@            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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