only build with JDK17

JDK20 not supported (yet?) in Github actions

Author: Gubaer

.github/workflows/gradle.yml

@@ -27,7 +27,6 @@ jobs:
           - release
         jdk:
           - 17
-          - 20
 
     name: Build plugin on ${{ matrix.platform }} - JDK ${{ matrix.jdk }}, minJosmVersion=${{ matrix.josm-version }}
     runs-on: ${{ matrix.platform }}