Rename maintenance-3.0.x and clear 3.1.x artefacts (phase 2)

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

1. Check with the Platform team that no addons rely on 3.1.x anymore. Delete all 3.1.x release candidate NPM packages.
2. Check with NOS team about the removal of the 11.x target platform. Once removed, delete all 3.1.x release candidate marketplace packages from connect and remove designer registries for 11.x (see NXS-6547). This depends on NXS-6524.
3. Rename the maintenance-3.0.x branch to 3.0 in both nuxeo-web-ui and nuxeo-elements repositories. 
4. Update legacy web ui pipeline and designer catalog job to fetch from the 3.0 branch. 
5. Update the Cowdin sync jobs for nuxeo-web-ui and nuxeo-elements so that they sync with the 3.0 branch. 
6. Delete master branch.
7. Delete versions 3.1.x and 3.x on Jira.
8. Send email notifying of the branch rename and the deletion of the artifacts.
9. (bonus) Delete old features branches to reduce clutter when searching for 3.0.