update to actions/cache@v4

Author: toKrause

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
 
     - name: Fetch Geckodriver cache
       id: geckodriver-cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ~/geckodriver
         key: ${{ env.GECKODRIVER_VERSION }}
@@ -39,7 +39,7 @@ jobs:
 
     - name: Fetch Bibutils cache
       id: bibutils-cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ~/bibutils
         key: ${{ env.BIBUTILS_VERSION }}