matrix-synapse: 1.3.1 -> 1.4.0

Bumps `matrix-synapse` to version 1.4.0[1]. With this version the
following changes in the matrix-synapse module were needed:

* Removed `trusted_third_party_id_servers`: option is marked as deprecated
  and ignored by matrix-synapse[2].
* Added `account_threepid_delegates` options as replacement for 3rdparty
  server features[3].
* Added `redaction_retention_period` option to configure how long
  redacted options should be kept in the database.
* Added `ma27` as maintainer for `matrix-synapse`.

Co-Authored-By: Notkea <pacien@users.noreply.github.com>
Co-authored-by: Maximilian Bosch <maximilian@mbosch.me>

[1] https://matrix.org/blog/2019/10/03/synapse-1-4-0-released
[2] https://github.com/matrix-org/synapse/pull/5875
[3] https://github.com/matrix-org/synapse/pull/5876