Changes to the branching workflow

[Leo Famulari]

Based on experiences with the last "staging" cycle and discussions at
the most recent Guix Day meeting [0], we've changed the branching
workflow.

The default branch names remain "core-updates" and "staging".

When we begin actively building and testing the branches, they will be
renamed to "core-updates-frozen" and "staging-frozen", respectively.

This will indicate that they are closed to any changes except for bug
fixes and merges of the master branch.

During those periods, new patches can be pushed to "core-updates-next"
and "staging-next".

Hopefully these changes will clarify the status of the branches and
reduce confusion.

[0] https://lists.gnu.org/archive/html/guix-devel/2021-02/msg00163.html