Accept arbitrary capitalization for coverage build type (backport #435)

[mergify]

🦟 Bug fix

Summary

When running cmake -DCMAKE_BUILD_TYPE=coverage (which most of our CI does), gz-cmake generates a warning

Code coverage results with an optimized (non-Debug) build may be misleading

This is only because we are specifically looking for Coverage, with uppercase C. This patch makes it possible to pass any capitalization of the word.

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.

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This is an automatic backport of pull request #435 done by [Mergify](https://mergify.com).