HHH-13925 Upgrade to Gradle 6.3

[yrodiere]

https://hibernate.atlassian.net/browse/HHH-13925

This is a preview: I ran the build locally to some extent, but I'd like a build from the CI. Also, I still need to run other builds, especially on JDK13 and 14, and to test publishing. => Done (see comments)

[yrodiere]

Everything seems in order:

• The CI build for JDK8 succeeded.
• The CI build for JDK11 failed on this PR, but that's a transient failure caused by problems transfering Maven artifacts. I relaunched the build.
• The CI build for JDK13 succeeded: https://ci.hibernate.org/blue/organizations/jenkins/hibernate-orm-personal-yoann-jdk13/detail/hibernate-orm-personal-yoann-jdk13/6/pipeline
• The CI build for JDK14 succeeded: https://ci.hibernate.org/blue/organizations/jenkins/hibernate-orm-personal-yoann-jdk14/detail/hibernate-orm-personal-yoann-jdk14/2/pipeline
• I managed to publish to my local Maven repository successfully
• I managed to publish artifacts to bintray successfully: https://bintray.com/yrodiere/hibernate-release-testing/hibernate-orm/999.4.0.Final
• I tested the hibernate-gradle-plugin with an application using Gradle 4.4: it still works, despite the plugin being compiled against Gradle 6 APIs.

[sebersole]

Overall I thought it looks fine. I only looked closely through the main commit, skipping all of the "clean up" commits. I would like to see the headers changed as mentioned

Quoting: "No problem then"

[yrodiere]

Merged, thanks!