* Add matrix_bridges_msc4190_enabled flag for using msc4190 on supported mautrix bridges.

* Apply to_json to msc4190 in mautrix configs

* Add | to_json to mautrix bridge registration io.element.msc4190.

* require matrix_synapse_experimental_features_msc3202_device_masquerading_enabled for matrix_bridges_msc4190_enabled

* Also add msc4190 support for mautrix-telegram

Now that https://github.com/element-hq/element-web/pull/29471
is part of v1.11.96, we don't need to carry around this custom workaround.

This also reverts e5574a405e because:
- it was causing issues on some servers (not clear why)
- such workarounds are no longer necessary when doing multi-stage building.

This is a security patch for Synapse that fixes an actively exploited bug.