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.

Element web : update welcome page template

Add support for postgres 13

I don't believe Ansible v1 would even go as far as executing this
sanity check, but.. Adding an extra defensive check for completeness.

Client well known compatibility

Related to #658 (Github Issue).

Related to #658 (Github Issue).

According to the docs, "e2ee" is already under "io.element":
  https://github.com/vector-im/element-web/blob/develop/docs/e2ee.md#disabling-encryption-by-default
however "jitsi" is still under "im.vector.riot":
  https://github.com/vector-im/element-web/blob/develop/docs/jitsi.md#configuring-element-to-use-your-self-hosted-jitsi-server

For now let's just maintain backward and forward compatibility for both
settings since the client version is out of the control of this
playbook.

Allow configuration of Element's bug_report_endpoint_url and showLabsSettings

showLabsSettings is the new enableLabs I guess. enableLabs doesn't seem to do anything anymore. It had been deprecated for a while.

This PR also removes @riot-bot:matrix.org as the default welcome_user_id since it doesn't exist anymore.