Fix run-in-docker.sh for JDK 11+ (OpenAPITools#15655)

OpenAPI Generator 7 requires JDK 11+, but the `run-in-docker.sh` had not
been updated and was still using JDK 8.

Author: halostatue

run-in-docker.sh

@@ -21,4 +21,4 @@ docker run --rm -it \
         -v "${PWD}/CI/run-in-docker-settings.xml:/var/maven/.m2/settings.xml" \
         -v "${maven_cache_repo}:/var/maven/.m2/repository" \
         --entrypoint /gen/docker-entrypoint.sh \
-        maven:3-jdk-8 "$@"
+        maven:3-jdk-11 "$@"