Upgrade versions in GitHub Actions

Fixes a bunch of deprecation warnings.

.github/workflows/build.yml

@@ -10,16 +10,16 @@ jobs:
 
     steps:
       - name: 'Checkout source code'
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: 'Set up JDK'
-        uses: actions/setup-java@v2
+        uses: actions/setup-java@v3
         with:
           java-version: '11'
           distribution: 'adopt'
 
       - name: 'Cache dependencies'
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: ~/.m2
           key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
@@ -29,7 +29,7 @@ jobs:
         run: mvn -B package --file pom.xml
 
       - name: 'Upload artifact'
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v3
         with:
           name: MobArena.jar
           path: target/MobArena.jar