Use jdk17 for 2.x builds

[srstsavage]

Currently the 2.28.x image has Java 11, which causes the webapp to fail to deploy because GeoServer 2.28.x requires Java 17.

$ docker pull docker.osgeo.org/geoserver:2.28.x         
2.28.x: Pulling from geoserver
Digest: sha256:687b2d8402cbbccf375a485a8df9c6ef4c45cc320edba8b0c92f0e0173b6275f
Status: Image is up to date for docker.osgeo.org/geoserver:2.28.x
docker.osgeo.org/geoserver:2.28.x
$ docker run --rm -it --entrypoint "" docker.osgeo.org/geoserver:2.28.x java -version
openjdk version "11.0.28" 2025-07-15
OpenJDK Runtime Environment Temurin-11.0.28+6 (build 11.0.28+6)
OpenJDK 64-Bit Server VM Temurin-11.0.28+6 (build 11.0.28+6, mixed mode, sharing)
$ GSID=$(docker run -d --name gs-2-28-x-test docker.osgeo.org/geoserver:2.28.x)      
$ docker exec $GSID grep UnsupportedClassVersionError /usr/local/tomcat/logs
grep: /usr/local/tomcat/logs: Is a directory
$ docker exec $GSID grep -R UnsupportedClassVersionError /usr/local/tomcat/logs
/usr/local/tomcat/logs/localhost.2025-11-03.log:	java.lang.UnsupportedClassVersionError: org/geoserver/GeoserverInitStartupListener has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 55.0 (unable to load class [org.geoserver.GeoserverInitStartupListener])
/usr/local/tomcat/logs/localhost.2025-11-03.log:	java.lang.UnsupportedClassVersionError: org/geoserver/logging/LoggingStartupContextListener has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 55.0 (unable to load class [org.geoserver.logging.LoggingStartupContextListener])
/usr/local/tomcat/logs/localhost.2025-11-03.log:	java.lang.UnsupportedClassVersionError: org/geoserver/platform/GeoServerContextLoaderListener has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 55.0 (unable to load class [org.geoserver.platform.GeoServerContextLoaderListener])
/usr/local/tomcat/logs/localhost.2025-11-03.log:	java.lang.UnsupportedClassVersionError: org/geoserver/platform/GeoServerHttpSessionListenerProxy has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 55.0 (unable to load class [org.geoserver.platform.GeoServerHttpSessionListenerProxy])