ci: update actions/cache to v4

Reference: https://github.blog/changelog/2024-12-05-notice-of-upcoming-releases-and-breaking-changes-for-github-actions/#actions-cache-v1-v2-and-actions-toolkit-cache-package-closing-down

Author: darrachequesne

.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