Bumping CI Java version to Java 15.

Author: Craigacp

.github/workflows/maven.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       matrix:
         # test against supported LTS versions and latest
-        java: [ 8, 11, 14 ]
+        java: [ 8, 11, 15 ]
     name: OLCUT - OpenJDK ${{ matrix.java }}
     steps:
       - uses: actions/checkout@v2