reverted back to JDK15 build

JDK17 build needs upgrade of Mockito and related code changes

Author: chrjohn

.github/workflows/maven.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-18.04, macOS-latest, windows-2016]
-        java: [8, 11, 17]
+        java: [8, 11, 15]
       fail-fast: false
       max-parallel: 4
     name: Test JDK ${{ matrix.java }}, ${{ matrix.os }}