iwyu.yml: reverted to Clang 21 for now [skip ci] (danmar#8318)

Author: firewave

.github/workflows/iwyu.yml

@@ -217,13 +217,13 @@ jobs:
           sudo apt-get purge --auto-remove llvm python3-lldb-14 llvm-14
           wget https://apt.llvm.org/llvm.sh
           chmod +x llvm.sh
-          sudo ./llvm.sh 22
-          sudo apt-get install -y clang-tools-22
+          sudo ./llvm.sh 21
+          sudo apt-get install -y clang-tools-21
 
       - name: Install libc++
         if: matrix.stdlib == 'libc++'
         run: |
-          sudo apt-get install -y libc++-22-dev
+          sudo apt-get install -y libc++-21-dev
 
       - name: Install Qt ${{ env.QT_VERSION }}
         uses: jurplel/install-qt-action@v4
@@ -238,8 +238,8 @@ jobs:
         run: |
           cmake -S . -B cmake.output -Werror=dev -DCMAKE_BUILD_TYPE=Release -DHAVE_RULES=On -DBUILD_TESTING=On -DBUILD_GUI=On -DWITH_QCHART=On -DBUILD_TRIAGE=On -DENABLE_CHECK_INTERNAL=On -DCMAKE_GLOBAL_AUTOGEN_TARGET=On -DDISABLE_DMAKE=On -DCMAKE_DISABLE_PRECOMPILE_HEADERS=On -DCPPCHK_GLIBCXX_DEBUG=Off -DUSE_MATCHCOMPILER=Off -DEXTERNALS_AS_SYSTEM=On -DUSE_LIBCXX=${{ matrix.use_libcxx }}
         env:
-          CC: clang-22
-          CXX: clang++-22
+          CC: clang-21
+          CXX: clang++-21
 
       - name: Prepare CMake dependencies
         run: |
@@ -256,7 +256,7 @@ jobs:
       - name: clang-include-cleaner
         run: |
           # TODO: run multi-threaded
-          find $PWD/cli $PWD/lib $PWD/test $PWD/gui -maxdepth 1 -name "*.cpp" | xargs -t -n 1 clang-include-cleaner-22 --print=changes --extra-arg=-w --extra-arg=-stdlib=${{ matrix.stdlib }} -p cmake.output > clang-include-cleaner.log 2>&1
+          find $PWD/cli $PWD/lib $PWD/test $PWD/gui -maxdepth 1 -name "*.cpp" | xargs -t -n 1 clang-include-cleaner-21 --print=changes --extra-arg=-w --extra-arg=-stdlib=${{ matrix.stdlib }} -p cmake.output > clang-include-cleaner.log 2>&1
 
       - uses: actions/upload-artifact@v4
         if: success() || failure()