Use v2-preview of upload-action to upload the jars again

Author: TBlueF

.github/workflows/gradle.yml

@@ -15,3 +15,7 @@ jobs:
       run: ./gradlew test
     - name: Build with Gradle
       run: ./gradlew build
+    - uses: actions/upload-artifact@v2-preview
+      with:
+        name: artifact
+        path: build/release/*