This:

- brings consistency - no more mixing `_name_prefix` and `_registry_prefix`
- adds extensibility - a future patch will allow reconfiguring all registry prefixes for all roles in the playbook

We still have `_docker_` vs `_container_` inconsistencies.
These may be worked on later.

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3409
- https://github.com/t2bot/matrix-media-repo/releases/tag/v1.3.5
- https://github.com/matrix-org/matrix-spec-proposals/pull/3916

Support for authenticated media routes is enabled by default, but
variables are in place to disable it if necessary.

This change has not been tested.

`matrix-media-repo` is the only role that seems incompatible with the
changes introduced by Traefik v3, due to its use of `PathPrefix` with
regular expressions in a few places.

Regular expressions should now be used with `PathRegexp`, not
`PathPrefix`. Furthermore, they should follow the Golang regexp syntax,
as described in the migration guide:
https://doc.traefik.io/traefik/migration/v2-to-v3-details/#dynamic-configuration-changes

This should come (and already does) from group_vars/matrix_servers