Heroku: using JDK 16 now, also going to 16 in GHA workflow

Author: jonashackt

.github/workflows/build.yml

@@ -8,7 +8,7 @@ jobs:
 
     strategy:
       matrix:
-        java-version: [ 8, 11, 15 ]
+        java-version: [ 8, 11, 16 ]
 
     steps:
       - uses: actions/checkout@v2
@@ -27,7 +27,7 @@ jobs:
       - uses: actions/setup-java@v2
         with:
           distribution: 'adopt'
-          java-version: 15
+          java-version: 16
       - run: mvn -B verify --no-transfer-progress
 
       - uses: codecov/codecov-action@v1

system.properties

@@ -0,0 +1 @@
+java.runtime.version=16