Create a master-2021 branch aligned with Nuxeo Platform 2021 LTS

The target platforms are: 10.10 LTS, 2021 LTS, 11.x FT

We currently develop:

• version 2.x on master-10.10 for Nuxeo 10.10 LTS

• version 3.x on master for Nuxeo 11.x FT

We need a branch master-2021 for Nuxeo 2021 LTS with version 3.0-2021.

This branch should be kept aligned with master as long as possible.

When master will start using 11.x features not available on 2021, then the master-2021 branch will have to be maintained like master-10.10.
Until that divergence, we should be able to automatically cherry-pick the commits.
When that will happen, the master version must be major increased. Until that, master and master-2021 should be able to share the same version modulo the 2021 suffix.