Ubuntu 18.04 requires actions/chevkout v3

Author: ckormanyos

.github/workflows/ci.yml

@@ -480,7 +480,7 @@ jobs:
     runs-on: ${{matrix.os}}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v3
 
       - name: Setup Boost
         shell: cmd