for matrix_nginx_proxy_proxy_synapse_metrics_basic_auth_key
resolves #1308

Seems like Ubuntu 18.04 does not have a dedicated `systemd-timesyncd` package, nor
does it include the `systemd-timesyncd` binary in the main `systemd` package.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1302

Regression since https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1192

#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)

From 6f80292745

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1287
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1288

.. but does it in a more complete way

Upgrade mjolnir (v0.1.19 -> v1.1.20)

Remark: interesting version number increase :-)

Update release (release-0.30.0 -> release-0.31.0)

See:
- https://github.com/vector-im/element-web/releases/tag/v1.8.4
- https://matrix.org/blog/2021/09/10/pre-disclosure-upcoming-critical-fix-for-several-popular-matrix-clients

This reverts commit e913347fe1.