Fix CMake warnings in Noble

[Blast545]

🦟 Bug fix

Fixes CMake warnings in Noble

Summary

Removing the deprecated Dart (Testing tool) include and explicitly setting the timestamp option when fetch declaring googleTest in a specific example.

Reference build: https://build.osrfoundation.org/job/gz_sim-ci-gz-sim8-noble-amd64/18/

Checklist

• [x] 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.

[azeey]

https://github.com/gazebosim/gz-sim/pull/2387 is close to being merged and it has changes to examples/standalone/gtest_setup/CMakeLists.txt. Should we wait for that?

[Blast545]

@azeey I think if that one gets merged, this PR won't be needed, as the complete FetchContent logic is removed.

I didn't saw that one, if that's close to be merged I'll just put this one as a draft and close/continue if needed be.

[Blast545]

@ros-pull-request-builder retest this please