Add clang 21 to linux CI and update actions/checkout (#2869)

# Checklist

- [x] The title and commit message(s) are descriptive.
- [ ] Small commits made to fix your PR have been squashed to avoid
history pollution.
- [ ] Tests have been added for new features or bug fixes.
- [ ] API of new functions and classes are documented.

# Description

Update the github action actions/checkout to v4 and add clang 21

Author: Alex-PLACET

.github/workflows/linux.yml

@@ -22,6 +22,7 @@ jobs:
         - {compiler: clang, version: '18', name: column-major, flags: -DDEFAULT_COLUMN_MAJOR=ON}
         - {compiler: clang, version: '19', name: assert, flags: -DXTENSOR_ENABLE_ASSERT=ON}
         - {compiler: clang, version: '20', name: column-major, flags: -DDEFAULT_COLUMN_MAJOR=ON}
+        - {compiler: clang, version: '21', name: assert, flags: -DXTENSOR_ENABLE_ASSERT=ON}
         - {compiler: gcc, version: '11', name: openmp, flags: -DXTENSOR_USE_OPENMP=ON}
         - {compiler: gcc, version: '11', name: noexcept, flags: -DXTENSOR_DISABLE_EXCEPTIONS=ON}
         - {compiler: gcc, version: '12', name: xsimd, flags: -DXTENSOR_USE_XSIMD=ON}
@@ -44,15 +45,15 @@ jobs:
         chmod +x llvm.sh
         sudo ./llvm.sh ${{matrix.sys.version}}
         sudo apt-get install -y clang-tools-${{matrix.sys.version}}
-        sudo update-alternatives --install /usr/bin/clang++                 clang++                     /usr/bin/clang++-${{matrix.sys.version}} 200
-        sudo update-alternatives --install /usr/bin/clang                       clang                         /usr/bin/clang-${{matrix.sys.version}} 200
-        sudo update-alternatives --install /usr/bin/clang-scan-deps   clang-scan-deps     /usr/bin/clang-scan-deps-${{matrix.sys.version}} 200
+        sudo update-alternatives --install /usr/bin/clang++         clang++             /usr/bin/clang++-${{matrix.sys.version}} 200
+        sudo update-alternatives --install /usr/bin/clang           clang               /usr/bin/clang-${{matrix.sys.version}} 200
+        sudo update-alternatives --install /usr/bin/clang-scan-deps clang-scan-deps     /usr/bin/clang-scan-deps-${{matrix.sys.version}} 200
         sudo update-alternatives --set clang /usr/bin/clang-${{matrix.sys.version}}
         sudo update-alternatives --set clang++ /usr/bin/clang++-${{matrix.sys.version}}
         sudo update-alternatives --set clang-scan-deps /usr/bin/clang-scan-deps-${{matrix.sys.version}}
 
     - name: Checkout code
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
 
     - name: Set conda environment
       uses: mamba-org/setup-micromamba@main
@@ -75,7 +76,7 @@ jobs:
 
     - name: Build
       working-directory: build
-      run: cmake --build . --target test_xtensor_lib --parallel 8
+      run: cmake --build . --target test_xtensor_lib
 
     - name: Run tests
       working-directory: build

.github/workflows/osx.yml

@@ -25,7 +25,7 @@ jobs:
     steps:
 
     - name: Checkout code
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
 
     - name: Set conda environment
       uses: mamba-org/setup-micromamba@main

.github/workflows/windows.yml

@@ -36,7 +36,7 @@ jobs:
         echo "CXX=clang++" >> $GITHUB_ENV
 
     - name: Checkout code
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
 
     - name: Set conda environment
       uses: mamba-org/setup-micromamba@main