[Java] Build with Java 15.

Author: mjpt777

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
  strategy:
  fail-fast: false
  matrix:
- java: [ '8', '11', '14' ]
+ java: [ '8', '11', '15' ]
  steps:
  - name: Checkout code
  uses: actions/checkout@v2