Fixup for f97e849018

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3354

Continuation of 9f2eff2ac7

Provoked by 7749048bf8
(https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3353)

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/3349

`docker.io/turt2live/matrix-media-repo:v1.3.4` is amd64-only.

`ghcr.io/t2bot/matrix-media-repo:v1.3.4` is a multi-arch image which
works on arm64.

commit cf8637efac
Author: Slavi Pantaleev <slavi@devture.com>
Date:   Sun Mar 24 19:14:57 2024 +0200

    Make devture_systemd_docker_base_ipv6_enabled automatically reconfigure geerlingguy/ansible-role-docker

    Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3218

commit dc7af3bc7d
Author: Slavi Pantaleev <slavi@devture.com>
Date:   Sun Mar 24 19:10:31 2024 +0200

    Replace matrix_ipv6_enabled with devture_systemd_docker_base_ipv6_enabled

    Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3218

commit 07e900d6a2
Author: Slavi Pantaleev <slavi@devture.com>
Date:   Sun Mar 24 19:01:51 2024 +0200

    Improve matrix_ipv6_enabled comments

commit 3f03ca7f69
Author: Tilo Spannagel <development@tilosp.de>
Date:   Sat Mar 9 19:27:50 2024 +0000

    Add setting to enable ipv6

This moves the comments from being just in Jinja,
to actually ending up in the generated `labels` file,
which makes inspection of the final result easier.

Also, some new lines were added here and there to make labels
more legible.

The generated file may still include weird new-lines due to
various `if` statements yielding content or not, but that's not so ugly
anymore - now that we have proper start/end sections that are visible in
the final `labels` file.

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

`matrix_media_repo_identifier` is already defined in the role defaults,
which is a better role to have it anyway.

* Updated MMR docs with updated fields in main.yaml

* Removed uneeded placeholder db password

This is backward-compatible with what we had before. We're not changing
the SSL mode - just making it configurable.

Most components are defaulting to `sslmode=disable`, while some
(`matrix-bot-matrix-reminder-bot` and others) do not specify an `sslmode` at all.

We're making sslmode configurable, because certain external Postgres
servers may be configured to require SSL encryption.
In such cases `sslmode=disable` does not work and needs to be changed to
`sslmode=require` or something else (`verify-ca`, `verify-full`, etc).