This also updates validation tasks and documentation, pointing to
variables in the matrix-synapse role which don't currently exist yet
(e.g. `matrix_synapse_container_labels_client_synapse_admin_api_enabled`).

These variables will be added soon, as Traefik labels are added to the
`matrix-synapse` role. At that point, the `matrix-synapse-reverse-proxy-companion` role
will be updated to also use them.

Related to https://github.com/matrix-org/synapse/pull/16265

Related to 0f02f0b4da

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2656
- e4a25d022c

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2656
- 4af0aec54d

Related to:

- e55269e14f
- 3d70cc393f
- 4fc85e5a92

Related to:

- 682d31c702
- ecbe0ddbe7
- f4fc83ac75

.. and https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2569

Related to 8f10c8b054

This was useful when the order of these roles in relation to Synapse
mattered (when we were injecting stuff into Synapse variables during
runtime). This is no longer the case since 0ea7cb5d1812a88, so all of
this can be removed.

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.