Replace GitHub build for Java 15 with Java 16 and drop Java 16-ea.

Add Java 17-ea.
diff --git a/.github/workflows/maven.yml b/.github/workflows/maven.yml
index 55cd006..2042534 100644
--- a/.github/workflows/maven.yml
+++ b/.github/workflows/maven.yml
@@ -24,10 +24,10 @@
     continue-on-error: ${{ matrix.experimental }}
     strategy:
       matrix:
-        java: [ 8, 11, 15 ]
+        java: [ 8, 11, 16 ]
         experimental: [false]
         include:
-          - java: 16-ea
+          - java: 17-ea
             experimental: true        
         
     steps: