Migration of JGit and EGit to GerritHub/GitHub

As already discussed in #680 the JGit and EGit projects would like to migrate their repositories to GerritHub which is a mirroring solution between Gerrit hosted by GerritForge and GitHub which enables using Gerrit for code review and GitHub for browsing repos and issue tracking. The solution uses GitHub authentication also for Gerrit.

Luca Milanesio from GerritForge has added another site to their multi-site Gerrit deployment to prepare this.

Gerrit 3.7 was enhanced to allow importing changes including review metadata from another Gerrit server. This way we can avoid loosing valuable review discussions.

There are a couple of open questions to prepare the migration:

• Luca proposed to use the domain name eclipse.gerrithub.io for the Gerrit site he added for hosting Eclipse projects. Do you approve usage of the name eclipse in this domain name ? Only repos of Eclipse projects would be hosted under this domain.
• The ECA plugin will be deployed on GerritHub to enable checking if contributors have signed the ECA. Will you allow the ECA plugin deployed on GerritHub to connect to the foundation's API for checking if a user has signed the ECA ?
• We will enhance the ECA plugin to allow project specific configuration, this will enable deploying it across all Gerrit sites of GerritHub's multi-site deployment. This will allow failover to another Gerrit primary in case of availability issues without forcing other projects hosted on GerritHub to also use the ECA plugin.
• We intend to continue using the JGit and EGit CI instances https://ci.eclipse.org/jgit and https://ci.eclipse.org/egit

We propose to schedule the migration after the 2023-06 simultaneous release (June 14) in order to avoid any interference.