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 is an attempt at optimizing service startup.

The effect is most pronounced when many services are restarted one by one.
The systemd service manager role sometimes does this - for example when `just install-service synapse` runs.
In such cases, a 5-second delay for each Synapse worker service
(or other bridge/bot service that waits on the homeserver) quickly adds up to a lot.

When services are all stopped fully and then started, the effect is not so pronounced, because
`matrix-synapse.service` starts first and pulls all worker services (defined as `Wants=` for it).
Later on, when the systemd service manager role "starts" these worker services, they're started already.
Even if they had a 5-second wait each, it would have happened in parallel.