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

This reverts commit f7e34671c3d1a8b590b8ee8d0ff82b62579d80fa.
diff --git a/.github/workflows/maven.yml b/.github/workflows/maven.yml
index cddb676..9bd42c0 100644
--- a/.github/workflows/maven.yml
+++ b/.github/workflows/maven.yml
@@ -24,7 +24,7 @@
     continue-on-error: ${{ matrix.experimental }}
     strategy:
       matrix:
-        java: [ 8, 11, 14 ]
+        java: [ 8, 11, 15 ]
         experimental: [false]
         include:
           - java: 16-ea