gearman-plugin-0.6.0

• Require latest LTS 2.239.1 to take in account the master node being renamed built-in node. See 2.319.X Upgrade guide -
• JENKINS-54888 potentially address a deadlock due to SaveableListener being invoked while extensions are loaded. The SaveableListener.onChange() has been made to skip unless Jenkins has reached InitMilestone.COMPLETED

Internal

• Update repo.jenkins-ci.org to use https
• Introduce maven wrapper to control the version of Maven being used (3.8.4). Run ./mvnw instead of mvn