Update Java version used by Docker

In 6b2fd5a the java version used to build baritone
was bumped from 17 to 21, but the Docker build was not updated accordingly.

Dockerfile

@@ -5,7 +5,7 @@ ENV DEBIAN_FRONTEND noninteractive
 RUN apt update -y
 
 RUN apt install \
-          openjdk-17-jdk \
+          openjdk-21-jdk \
 	  git \
           --assume-yes