Bump actions/checkout from 2 to 4 (#11)

Author: luraess

.github/workflows/CI.yml

@@ -12,7 +12,7 @@ jobs:
         os: [ubuntu-latest]
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v1
         with:
           version: ${{ matrix.julia-version }}