CI: Update LLVM/Clang versions to 21

[Explorer09]

This branch is for @natoscott who wants to look at all protential build problems with Clang 21, especially the -Wshorten-64-to-32 warnings that are left unfixed.

(Related to: #1673)

[Explorer09]

With the BSD version bump in PR #1822, the commit that explicitly enables -Wshorten-64-to-32 in BSDs (75dff70d8a6ec8978ac148e9a3450e665ace16cf) is no longer needed and can be dropped.