Update actions/checkout to v4

.github/workflows/build-centos7.yaml

@@ -23,7 +23,7 @@ jobs:
 
       - name: Update and install build tools
         # Add python-devel to support python in gdb.
-        # actions/checkout@v3 requires git >= 2.18.
+        # actions/checkout@v4 prefers (?) git >= 2.18.
         # mirror.centos.org doens't exist anymore after CentOS 7 EOL.
         run: |
           sed -i s/mirror.centos.org/vault.centos.org/g /etc/yum.repos.d/*.repo
@@ -49,7 +49,7 @@ jobs:
             ./cmake.sh --skip-license --prefix=/usr && \
             rm -f cmake.sh
 
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - name: Checkout submodules
         run: |