upgrade to openjdk ver. 18

Dockerfile

 # Base image on OpenJDK 11 JRE
-FROM openjdk:16.0.1-slim
+FROM openjdk:18-slim
 LABEL org.label-schema.name="clspigot-docker" \
       org.label-schema.description="Docker Container for an Papermc Server" \
       org.label-schema.vcs-url="https://gitlab.cl.uni-heidelberg.de/tmueller/clspigot-docker/" \