C++: Update PreprocBlock.qll QLDoc and add C++23 test

[jketema]

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• All new queries have appropriate .qhelp. See the documentation in this repository.
• QL tests are added if necessary. See Testing custom queries in the GitHub documentation.
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Files not reviewed (4)

• cpp/ql/lib/semmle/code/cpp/headers/PreprocBlock.qll: Language not supported
• cpp/ql/test/library-tests/headers/preprocBlock/preprocblock.expected: Language not supported
• cpp/ql/test/library-tests/headers/preprocBlock/preprocblock23.cpp: Language not supported
• cpp/ql/test/library-tests/headers/preprocBlock/preprocinclude.expected: Language not supported

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[geoffw0]

LGTM, thanks for updating this.

[jketema]

LGTM, thanks for updating this.

You're welcome, and thanks for the review.