We'be already been going against upstream defaults and have been
enabling backfilling for a few other bridges (Messenger, Instagram, Telegram, Twitter).

Now I'm enabling backfilling by default for the remaining ones, for
consistency.

While working on upgrading the Meta bridges to bridgev2, I've noticed
that {% raw %} and {% endraw %} on lines like that (immediately
preceding `username_template` may cause YAML indentation issues.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3479#issuecomment-2294956958

Since upgrading mautrix-slack (and pinning to v0.1.0) in e4b54c37fe258b17f49dd7ed58a18ef3abac1c41,
we expect double-puppeting to require the new appservice double-puppeting method.

This commit switches the mautrix-slack bridge to it.

Related to:
- https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3479
- https://github.com/mautrix/slack/releases/tag/v0.1.0
- https://mau.fi/blog/2024-08-mautrix-release/

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.

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

Update group_vars/matrix_servers

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

Remove old data migration tasks

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

Update roles/matrix-bridge-mautrix-discord/tasks/validate_config.yml

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

Redo bridge permissions