Make maintenance-3.0.x the main branch for Web UI and Elements (phase 1)

This is the first phase of the plan to phase out the master branch for Nuxeo Web UI and Nuxeo Elements. The work to be done under this ticket comprises of:

1. Backport any missing commits from master to the maintenance-3.0.x branch. We will have to manually compare the branches with git commands. We can also check on Jira for tickets pointing to 3.1.x or 3.x but not 3.0.x.
2. Backport promotion scripts.
3. Backport any missing labels. Simply copying over the message*.json files should be enough.
4. Turn the maintenance-3.0.x branch into the main branch of both nuxeo-web-ui and nuxeo-elements
5. Update the Cowdin sync jobs for nuxeo-web-ui and nuxeo-elements.
6. Delete web ui legacy pipeline for master (note that the owner of each copy of this pipeline will have to manually update their jobs)
7. Send email notifying that the master branch won't be used anymore, and that no more PR or commits should be open/pushed to that branch, and that the Jira versions 1.x and 3.x were delete and have no use. Warn that NPM and MP packages will be kept by a short period of time.
8. Update promotion scripts on both nuxeo-web-ui and nuxeo-elements to have the default branch pointing to maintenance-3.0.x.