[MAINT] Update SYCL_INCLUDE_DIR_HINT seach to include clang 21

[ndgrigorian]

This PR updates the search of possible clang versions for SYCL_INCLUDE_DIR_HINT in bld.bat to include clang 21

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[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[github-actions]

Array API standard conformance tests for dpctl=0.21.0dev0=py310h93fe807_121 ran successfully.
Passed: 1116
Failed: 5
Skipped: 119