ci: update actions/cache to v4

Author: darrachequesne

Diff for: .github/workflows/ci.yml

@@ -33,9 +33,9 @@ jobs:
           java-version: ${{ matrix.java }}
 
       - name: Cache Maven packages
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
-          path: ~/.m2
+          path: ~/.m2/repository
           key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
           restore-keys: ${{ runner.os }}-m2