default in encryption_enabled_by_default_for_room_type is misleading

This issue has been migrated from [#9794](https://github.com/matrix-org/synapse/issues/9794).

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### Description

The option `encryption_enabled_by_default_for_room_type` as introduced in #7639 is misleading.

A default is something that has a value as long as it's not overwritten. In this case it would mean, that encryption is enabled for the specified room type as long as it is not disabled for a specific room/client/...
The current implementation prevents creation of unencrypted DMs/rooms. This is clearly not a _default_, but _forced_ encryption.

### Expected Naming

`force_encryption_enabled_for_room_type` or similar.

### Version information

`v1.31.0` and current `develop` branch.