Bump clang-format to v21 (#634)

Author: jtraglia

.github/workflows/c-tests.yml

@@ -33,7 +33,7 @@ jobs:
         if: matrix.os == 'ubuntu-latest'
         uses: jidicula/clang-format-action@6cd220de46c89139a0365edae93eee8eb30ca8fe # v4.16.0
         with:
-          clang-format-version: '20'
+          clang-format-version: '21'
           check-path: 'src'
           exclude-regex: 'tinytest.h'