CI: switch to clang 21

.github/workflows/build.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       matrix:
         os:         [ubuntu-24.04]
-        compiler:   [g++-14, clang++-19, clang++-20]
+        compiler:   [g++-14, clang++-20, clang++-21]
         build_type: [Release, Debug]
         cache:      ['', Redis]
         include:
@@ -29,20 +29,20 @@ jobs:
         if: runner.os == 'Linux'
         run: |
           wget -O- https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-          if [[ ${{ matrix.compiler }} == "clang++-19" ]]; then
-            sudo add-apt-repository "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-19 main"
-          fi
           if [[ ${{ matrix.compiler }} == "clang++-20" ]]; then
+            sudo add-apt-repository "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-20 main"
+          fi
+          if [[ ${{ matrix.compiler }} == "clang++-21" ]]; then
             sudo add-apt-repository "deb http://apt.llvm.org/noble/ llvm-toolchain-noble main"
           fi
           sudo apt-get update
           sudo apt-get install z3 re2c ninja-build
-          if [[ ${{ matrix.compiler }} == "clang++-19" ]]; then
-            sudo apt-get install clang++-19
-          fi
           if [[ ${{ matrix.compiler }} == "clang++-20" ]]; then
             sudo apt-get install clang++-20
           fi
+          if [[ ${{ matrix.compiler }} == "clang++-21" ]]; then
+            sudo apt-get install clang++-21
+          fi
           if [[ "${{ matrix.cache }}" == "Redis" ]]; then
             sudo apt-get install libhiredis-dev
           fi