[ign -> gz] Migrate more internal variables

[chapulina]

• Part of CMake API from ign to gz gazebo-tooling/release-tools#719
• Follow up to [ign ➡️ gz] Internal functions #247

Summary

All these variables should be internal to gz-cmake, so they can be changed without tick-tocking. I verified that by building the entire stack on top of this branch, and also by searching for these variables in other libraries.

Test it

• CI should pass
• The entire stack should build locally on top of this branch

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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.

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