Updated to GHA checkout@v3 to avoid deprecation warnings.

.github/workflows/ci.yml

@@ -301,7 +301,7 @@ jobs:
             fi
             git config --global pack.threads 0
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install packages
         if: matrix.install
@@ -500,7 +500,7 @@ jobs:
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Setup Boost
         shell: cmd