Update Maven Version

[VerKWer]

Currently, following the build instructions

git clone git://github.com/jline/jline3.git
cd jline3
./build rebuild

leads to an error

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.4.1:enforce (enforce-maven) on project jline-parent: 
[ERROR] Rule 0: org.apache.maven.enforcer.rules.version.RequireMavenVersion failed with message:
[ERROR] Detected Maven Version: 3.5.3 is not in the allowed range [3.6.3,).

Bumping the Maven version in .mvn/wrapper/maven-wrapper.properties to the most recent stable release (3.9.6) solves the issue and as far as I could tell, there were no incompatibilities.