update pipelines, run on 16 (now GA) and drop 15

Author: LarsEckart

.github/workflows/test-windows.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        java: ['8', '11', '15', '16-ea', '17-ea']
+        java: ['8', '11', '16', '17-ea']
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: ['8', '11', '15', '16-ea', '17-ea']
+        java: ['8', '11', '16', '17-ea']
 
     steps:
     - uses: actions/checkout@v2