[change] Squash and Reduce Migrations for Faster Installation

[pandafy]

Is your change related to a problem? Please describe.

Currently, on a fresh installation of openwisp-radius, the process of applying migrations is slow due to the high number of migration files. This negatively impacts operational performance, especially for new deployments.

Describe the solution you'd like

Squash the existing migrations to minimize their total number while ensuring backward compatibility. The migration process for new installations should be significantly faster, without affecting upgrades from earlier versions. Update documentation if necessary to reflect these changes.

Additional context

Squashing migrations will streamline the installation process and enhance the user experience for new users of openwisp-radius.

[nemesifier]

We should do this only for the migrations that have been already released up to version 1.2.x.

We should also flag (eg: with comments) any complex data migration logic in openwisp_radius/migrations/__init__.py for removal.

In a future version we'll be then able to remove the non squashed migrations and the logic flagged for removal.

Also some test classes in test_migrations will have to be flagged for removal and removed in a future step.