Unhide warning when schema test is skipped

[scpeters]

🦟 Bug fix

Unhides a warning added in #1655

Summary

The schema integration test fails when run with very new versions of xmllint. This was discovered when adding CI with Ubuntu 26.04 in #1655, and logic to disable the test with a warning was added in that PR. Unfortunately, the warning is not currently being reported due to the order in which gz_build_warning is processed in gz_configure_build even though the test is skipped:

• https://build.osrfoundation.org/job/sdformat-ci-main-resolute-amd64/2/

This unhides the warning by switching to message(WARNING ""), in a similar manner to gazebosim/gz-transport#565.

Checklist

• Signed all commits for DCO
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• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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[scpeters]

From Ubuntu Resolute CI:

  CMake Warning at test/integration/CMakeLists.txt:89 (message):
    xmllint version is too new (21502).  schema_test won't be run

[scpeters]

if we want CI to ignore this as a warning, we can use message(STATUS instead