Fix CI: use JDK 21 for builds, upload-artifact@v3, chmod gradlew

.gitea/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
       - uses: actions/setup-java@v4
         with:
           distribution: temurin
-          java-version: '18'
+          java-version: '21'
           cache: maven
 
       - uses: stCarolas/setup-maven@v5
@@ -24,7 +24,7 @@ jobs:
       - name: Build with Maven
         run: mvn -B package
 
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v3
         with:
           name: TwitchAPI-jar
           path: target/*.jar