Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3088

Looks like the migration to the Go-based Signal bridge hardcoded the
`signalbot` username instead of using the variable we had.
Related to: https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3041

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3089

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3086

Such a core service probably belongs better when it's in the homeserver network

This is mostly so as to avoid referring to variables from other roles,
like `matrix_nginx_proxy_proxy_matrix_client_api_addr_with_container`.

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

ldap-registration-proxy is already connected to the homeserver
container's network by default (via group vars), so there's no need for this.

This also makes it handle the `/_matrix/client/v3/register` endpoint,
not just `/_matrix/client/r0/register`

This reverts commit bf95ad2235.

This was a bad idea.
It's better to have people manually define the password.

Otherwise, `matrix_homeserver_generic_secret_key` changing some day in
the future would break the bot and one would have to figure out how to
reset its password manually.

Using an explicit password is more stable.

Running just `setup-all,start` is not enough, because it doesn't run `ensure-matrix-users-created`
and the bot account won't get created.

* chore(deps): update signal bridge version + config

* style(deps): rename default note to self config variable

* Add to_json for additional safety

---------

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

This is split out from matrix-cactus-comments (see 241779b583),
but also heavily inspired by `matrix-static-files`.

The container image has only ever been available for amd64,
so not enabling self-building for the other architectures was a mistake
that orignally landed in:
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2089

Looks like these variables were originally named this way in
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2089

Related to https://github.com/mautrix/discord/issues/95