di: add JDK 15, drop JDK 14

Author: sullis

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [8, 11, 14]
+        java: [8, 11, 15]
     steps:
       - uses: actions/checkout@v2
       - uses: actions/cache@v2