Add keepalive on worker upstreams and use persistent connections

Fixes: https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3082

Related to: https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3079

Putting each client into its own network was good for isolation,
but it's quite wasteful in terms of the container network pool.

The latest tagged release (v0.1.3) does not pin any versions in its
Dockerfile and as such fails to build right now.

The `main` branch of rust-synapse-compress-state has already addressed
this and is buildable, but there's no tagged release yet.

Reported here: https://github.com/matrix-org/rust-synapse-compress-state/issues/134

This allows us to eliminate the companion and decrease overhead for
simple servers which do not use workers.

update mautrix-signal

This saves up 1 container network and avoids going through extra proxies
unnecessarily.

Downsides: decreasing security slightly due to less networking isolation

Benefits:

- decreased complexity
- having a generically-named `matrix-addons` network we may use for other things now (client apps, etc.)
- not exhausting the container networks pool with 2 (or more) networks and using just 1

chore(deps): update vectorim/element-web docker tag to v1.11.53

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.

matrix-nginx-proxy is going away and this is one of the features it
offered.

This feature will have no equivalent in our new Traefik-only
setup, although it's possible to implement it manually by using
`matrix_client_element_container_labels_additional_labels`

The spec had gotten updated

This is completely untested.

chore(deps): update halfshot/matrix-hookshot docker tag to v5.1.2

[#3074] Docker tag follows the version properly