updated cache to v4 in workflow

Author: Steve Hannah

.github/workflows/pr.yml

@@ -21,7 +21,7 @@ jobs:
         java-version: 1.8
         java-package: jdk+fx
     - name: Cache Maven dependencies
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ~/.m2
         key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}