Pin jitpack maven version compatible with the compiler plugin

The default maven version on jitpack (3.6.1) fails to build
commonmark-java 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

jitpack.yml

@@ -0,0 +1,2 @@
+before_install:
+  - sdk install maven 3.6.3