gh-152785: Upgrade LLVM to LLVM 21 in GitHub Action jobs (GH-152781)

Upgrade LLVM to LLVM 21 in GitHub Action jobs:

* Reusable Sanitizer
* Reusable Ubuntu ("Install Clang and BOLT")
(cherry picked from commit f5b3eef)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: vstinner

.github/workflows/reusable-san.yml

@@ -40,11 +40,11 @@ jobs:
         # Install clang
         wget https://apt.llvm.org/llvm.sh
         chmod +x llvm.sh
-        sudo ./llvm.sh 20
-        sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-20 100
-        sudo update-alternatives --set clang /usr/bin/clang-20
-        sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-20 100
-        sudo update-alternatives --set clang++ /usr/bin/clang++-20
+        sudo ./llvm.sh 21
+        sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-21 100
+        sudo update-alternatives --set clang /usr/bin/clang-21
+        sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-21 100
+        sudo update-alternatives --set clang++ /usr/bin/clang++-21
 
         if [ "${SANITIZER}" = "TSan" ]; then
           # Reduce ASLR to avoid TSan crashing

.github/workflows/reusable-ubuntu.yml

@@ -44,9 +44,9 @@ jobs:
     - name: Install Clang and BOLT
       if: ${{ fromJSON(inputs.bolt-optimizations) }}
       run: |
-        sudo bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)" ./llvm.sh 19
-        sudo apt-get install --no-install-recommends bolt-19
-        echo PATH="$(llvm-config-19 --bindir):$PATH" >> $GITHUB_ENV
+        sudo bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)" ./llvm.sh 21
+        sudo apt-get install --no-install-recommends bolt-21
+        echo PATH="$(llvm-config-21 --bindir):$PATH" >> $GITHUB_ENV
     - name: Configure OpenSSL env vars
       run: |
         echo "MULTISSL_DIR=${GITHUB_WORKSPACE}/multissl" >> "$GITHUB_ENV"