[CI] add CI matrix for clang-21

see also llvm/llvm-project#124870

Author: shyouhei

.github/workflows/compilers.yml

@@ -103,6 +103,7 @@ jobs:
       - uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938 # v4.2.0
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
+      - { uses: './.github/actions/compilers', name: 'clang 21', with: { tag: 'clang-21' } }
       - { uses: './.github/actions/compilers', name: 'clang 20', with: { tag: 'clang-20' } }
       - { uses: './.github/actions/compilers', name: 'clang 19', with: { tag: 'clang-19' } }
       - { uses: './.github/actions/compilers', name: 'clang 18', with: { tag: 'clang-18' } }