This mostly fixes `key-order` warnings around
`block` statements.

Reference: https://ansible-lint.readthedocs.io/en/latest/default_rules/#package-latest

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1931

* Implement MSC 1929 Homeserver Admin Contact
Fixes #1612

For details to the proposed (not accepted yet) MSC, see:
https://github.com/matrix-org/matrix-spec-proposals/blob/hs/proposal-admin-contact-1/proposals/1929-admin-contact.md

* Implement feedback from PR #1931

* Implement feedback from PR #1931

* Fix mixed indentation

Fedora 35 is:

- `ansible_os_family = 'RedHat'`
- `ansible_distribution_major_version = '35'`

Our RedHat checks against v7/v8 are really for RHEL derivatives (CentOS, Rockylinux,
AlmaLinux), but the same checks (by coincidence) apply for Fedora 35.

The problem is that `'35' > '7'` (comparing these as strings) is
`false`.

This patch makes sure that we always cast
`ansible_distribution_major_version` to an integer.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1610

Not hardcoding 'CentOS' and using the OS family ('RedHat') instead,
we now behave better on Rockylinux and AlmaLinux, etc.

With that said, we may or may not fully support CentOS/Rockylinux/AlmaLinux v8 yet.
Certain things were improved in
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/300.

v8 support is discussed here: https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/300
Certain things (firewalld?) may still be problematic. This patch does not try to address those.
If the remaining issues are confirmed to be fixed in the future, we can mark v8 as supported.

Discussed here: https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1510

We're trying to move away from implementation-specific variables,
hoping for a clean (implementation-neutral) examples/vars.yml file.

Doing this seems more reasonable and simpler.

The goal is to have a single variable which tells us which homeserver
software is in use. Much simpler than having if/elif/elif checks for
variables like (`matrix_synapse_enabled` and `matrix_dendrite_enabled`, etc.)
everywhere.

We recently had someone need to inject additional configuration into
`/.well-known/matrix/client` as described here:
22b245bbd1/docs/bigbluebutton.md (have-dimension-create-meetings-with-elements-video-call-button)

There may be other use cases as well.

#1192 lead to the following error for me on Archlinux:
`TASK [matrix-base : Install host dependencies] *******************************************************************************************************************************
fatal: [matrix.***.de]: FAILED! => changed=false 
  msg: |-
    failed to install systemd-timesyncd: error: target not found: systemd-timesyncd`

There is no package called `systemd-timesyncd` on Archlinux. The service is installed with the [`systemd`](https://archlinux.org/packages/core/x86_64/systemd/) package itself.

I suggest removing the `systemd-timesyncd` from 2453876eb9/roles/matrix-base/tasks/server_base/setup_archlinux.yml (L7)

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/963

This also simplifies Prerequisites, which is great.

It'd be nice if we were doing these checks in some optional manner
and reporting them as helpful messages (using
`matrix_playbook_runtime_results`), but that's more complicated.
I'd rather drop these checks completely.

Using the hardcoded IP did break while I was
messing with IPv6 stuff on the other branch

Future maintainer: check on https://docs.docker.com/engine/install/debian/ if Docker for
Debian 11 is released, then undo this commit

Fixup for https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/769

Related to 6e652e10ad

This also adds support for installing python3-docker (not python-docker)
in systems that run Python 3.

Raspbian doesn't seem to support arm64, so this is somewhat pointless
right now.

However, they might in the future. Doing this should also unify us
some more with `setup_debian.yml` with the ultimate goal of
eliminating `setup_raspbian.yml`.

It's likely installed by default, but it doesn't hurt to specify it.
It also makes us more the same with `setup_debian.yml`.

Well, `ubuntu` or `debian`, the same key is served right now,
so it doesn't really matter.

This seems cleaner and less prone to breakage though.

Until now, we've only supported non-amd64 on Raspbian.

Seems like there are now people running Debian/Ubuntu on ARM,
so we were forcing them into amd64 Docker packages.

I've gotten a report that this change fixes support
for Ubuntu Server 20.04 on RPi 4B.

The docker_network bug was fixed two years ago
This reverts commit 36658addcd.