Unhide cmake warning when Python3 Development is not found

[scpeters]

🦟 Bug fix

Unhides a cmake warning when python bindings will not be built

Summary

A cmake warning should be printed when python bindings will not be built (controlled by the logic around add_subdirectory(python)), but the warning was in a separate part of the code, allowing the warning to be missed if the Python3_Development_FOUND variable changed in between.

Since GZ_BUILD_WARNING("") only generates a cmake warning if invoked before gz_configure_build, I switched to use message(WARNING "") in e464330.

Also, the failed Ubuntu Noble CI build is expected because finding python is currently broken for all platforms, but only our Ubuntu Noble CI builds report cmake warnings. I believe finding python should be fixed once gazebosim/gz-msgs#479 is merged and released. I think it would be worth merging this so that our broken state is accurately reflected.

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)
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[scpeters]

this should be ported to gz-math and sdformat as well, since they use the same logic pattern