Gradle: Transfer the `gradle` image to Gradle

Moving the repository and updating to official maintainers from Gradle.

Signed-off-by: Christoph Obexer <[email protected]>

gradle/github-repo

@@ -1 +1 @@
-https://github.com/keeganwitt/docker-gradle
+https://github.com/gradle/docker-gradle

gradle/maintainer.md

@@ -1 +1 @@
-[Keegan Witt (of the Groovy Project)](%%GITHUB-REPO%%), [with the Gradle Project's approval](https://discuss.gradle.org/t/official-docker-images/21159/8)
+[Gradle, Inc.](%%GITHUB-REPO%%)