Fix JitPack builds

[zampino]

Jitpack is a useful maven repository to develop against unreleased code hosted on github. The default maven version on jitpack (3.6.1) fails to build commonmark-java main with

[ERROR] Failed to execute goal
org.apache.maven.plugins:maven-compiler-plugin:3.13.0:compile (default-compile) on project commonmark-test-util: The plugin
org.apache.maven.plugins:maven-compiler-plugin:3.13.0 requires Maven version 3.6.3

These changes ensure the maven version used by jitpack is compatible with the current maven-compiler-plugin.

Jitpack builds stopped working as of 682bdb09932a4846c389e9db43eba1b857e219cd, where the compiler plugin was last updated. I've also setup a mini repro in https://github.com/nextjournal/commonmark-java-jitpack-repro to test different versions against jitpack.