Merge tag 'v1.42.0rc2' into develop

Synapse 1.42.0rc2 (2021-09-06)
==============================

This version of Synapse removes deprecated room-management admin APIs, removes out-of-date
email pushers, and improves error handling for fallback templates for user-interactive
authentication. For more information on these points, server administrators are
encouraged to read [the upgrade notes](docs/upgrade.md#upgrading-to-v1420).

Features
--------

- Support room version 9 from [MSC3375](https://github.com/matrix-org/matrix-doc/pull/3375). ([\#10747](https://github.com/matrix-org/synapse/issues/10747))

Internal Changes
----------------

- Print a warning when using one of the deprecated `template_dir` settings. ([\#10768](https://github.com/matrix-org/synapse/issues/10768))