Closes #2981

Adds support for updating the gradle version used in a project via the gradle-wrapper.
See https://docs.gradle.org/current/userguide/gradle_wrapper.html

This reverts commit bc40d480.

Because of https://github.com/sindresorhus/got/issues/690

Change github redirect links from renovatebot.com/gh/ to togithub.com

feat: maven datasource

This reverts commit 9af3ef2e.

Helps with https://github.com/renovatebot/renovate/pull/3039 😄 

Closes #2069

Closes #3012