Switch to the new vectorim/element-web Docker image

This reverts commit 7d211b71e2.

The multi-arch images are now available, as discussed in #699 (Github Issue).

Fixes #696 (Github Issue)

We do this to match Synapse's new default "max_upload_size" (50MB).

This `matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb`
default value only affects standalone usage of the `matrix-nginx-proxy`
role. When the role is used in the context of the playbook,
the value is dynamically assigned from `group_vars/matrix_servers`.

Somewhat related to #692 (Github Issue).

The regex introduced in 63a49bb2dc seems to take precedence
over the bare location blocks, causing a regression.

> It is important to understand that, by default, Nginx will serve regular expression matches in preference to prefix matches.
> However, it evaluates prefix locations first, allowing for the administer to override this tendency by specifying locations using the = and ^~ modifiers.

Source: https://www.digitalocean.com/community/tutorials/understanding-nginx-server-and-location-block-selection-algorithms

Regression since 63a49bb2dc.

Related to #685 (Github Issue).

We now automatically expose the APIs when Synapse Admin is enabled.

While v1.22.0 supposedly has multi-arch Docker images
(thanks to https://github.com/matrix-org/synapse/pull/7921),
I can't them on Docker Hub yet, so I'm backing out of this change
for now and letting people fall back to self-building there.

Fix docker mount instruction in documentation

Fixes #685 (Github Issue).

The SSL certificates should be accessible to the nginx proxy container, not the synapse container.

add variables for secure_backup_required and secure_backup_setup_methods

Discussed in #669 (Github Issue).

Upgrade Element (1.7.8 -> 1.7.9)

Update nginx (1.19.2 -> 1.19.3)

Shameless refactor of all systemd descriptions for bridges

Upgrade Synapse (v1.20.1 -> v1.21.0)

Add docker prune

If the SQLite database was from an older version of Synapse, it appears
that Synapse would try to run migrations on it first, before importing.
This was failing, because the file wasn't writable.

Hopefully, this fixes the problem.

Adding '.python-version' to .gitignore

caddy2: removed unknown tls directive

Use up-to-date PostgreSQL container version for backups.

Related to 23daec748c and #662 (Github Issue).

Interestingly, no one has reported this failure before #662 (Github
Issue).

It doesn't make sense to keep saying that we support such old Ansible
versions, when we're not even testing on anything close to those.

Time is also passing and such versions are getting more and more
ancient. It's time we bumped our requirements to something that is more
likely to work.