Upgrading a public room erroneously makes it E2EE when E2EE is required for private rooms

This issue has been migrated from [#9246](https://github.com/matrix-org/synapse/issues/9246).

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### Description

(Note: I'm filing this issue on behalf of someone else, who can't currently file issues, so I don't have direct access to the homeserver in question.)

When Synapse is configured to require E2EE for private rooms, then when a public room is upgraded through that homeserver, it is erroneously marked as end-to-end encrypted. According to TravisR:

> looks like a synapse bug. encryption_enabled_by_default_for_room_type doesn't seem to work with room upgrades (/upgrade creates a room without a preset, which is by default a private room until it later updates the state)

Automatically enabling E2EE for the upgraded room should only occur for private rooms, instead.

### Steps to reproduce

- Run homeserver that requires E2EE for private rooms
- From an account on that homeserver, upgrade a public room, through `/upgraderoom 6` in Element Web/Desktop

### Version information

- **Homeserver**: privacytools.io

If not matrix.org:

- **Version**: unknown

- **Install method**: unknown

- **Platform**: unknown