Fature request: add a warning on clients that don't support knocking, when the room has join_rules knock or knock_restricted

[Mikaela]

Describe the bug

Matrix.to has ability to know the join_rules of a room (since matrix-org/matrix-spec-proposals#3266), but it doesn't actually warn when a client doesn't support the feature.

To Reproduce
Steps to reproduce the behavior:

1. Go to a matrix.to URI link which requires knocking e.g. https://matrix.to/#/%23space.piraatit.fi:matrix.org
2. Open in Element (top option)
3. In Element attempt to join room
4. Get error: you are not invited to room

Expected behavior

Matrix.to sees join_rules being knock or restricted_knock and adds a warning to clients that don't support knocking. Knocking is a part of Matrix specification since 1.1 and restricted_knock I hope to soon pass the MSC process.

Screenshots

Desktop (please complete the following information):

• OS: Fedora 36
• Browser Firefox
• Version 101.0b6 (64-bittinen)

Additional context

• https://spec.matrix.org/latest/client-server-api/#knocking-on-rooms