Update to gradle 6.7-rc-1 for test fixture. Build with Java 15 on CI.

.github/workflows/workflow.yml

@@ -47,10 +47,10 @@ jobs:
         npm run lint
         npm test
 
-    - name: Java 14
+    - name: Java 15
       uses: actions/setup-java@v1
       with:
-        java-version: 14
+        java-version: 15
     - uses: actions/cache@v2
       with:
         path: |