Finish spotless license header configuration (#1231)

* Update spotless and enable copyright header bits again

* Reformat square copyright headers

* Create separate format configuration for external files

* Update CI to use java 15 final

* include stacktrace in ci failures

* Update ktlint to 0.39

* Disable java formatting on JDK 15

* Fix google header

.github/workflows/build.yml

@@ -17,7 +17,7 @@ jobs:
           - 12
           - 13
           - 14
-          - 15-ea
+          - 15
 
     steps:
       - name: Checkout
@@ -42,7 +42,7 @@ jobs:
           java-version: ${{ matrix.java-version }}
 
       - name: Test
-        run: ./gradlew build check
+        run: ./gradlew build check --stacktrace
 
       - name: Publish (default branch only)
         if: github.ref == 'refs/heads/master' && matrix.java-version == '1.8'