Revert "Test on Java LTS 14 and remove Java 15"

This reverts commit f7e3467.

Author: aherbert

.github/workflows/maven.yml

@@ -24,7 +24,7 @@ jobs:
     continue-on-error: ${{ matrix.experimental }}
     strategy:
       matrix:
-        java: [ 8, 11, 14 ]
+        java: [ 8, 11, 15 ]
         experimental: [false]
         include:
           - java: 16-ea