Upgrading a room can orphan it from its space

### Steps to reproduce

1. As User A, create a space
2. Add Room A to the space
3. Invite User B to the space and Room A
4. Make User B an admin in Room A, but not the space
5. As User B, upgrade Room A to Room B
6. Invite User C to the space and Room B

For context, while this is an uncommon scenario, it's already happened in the Element Corp. space.

### Outcome

#### What did you expect?

User C should see Room B listed as a child of the space.

#### What happened instead?

To User C, Room B appears as an orphaned room, since the space actually only lists Room A as a child, and User C has no way to know about the upgrade.

Well, that's not entirely true. User C could find out about the upgrade by looking at the room creation event. But if the room were upgraded twice rather than once, this would still not be enough information to connect the upgraded room back to Room A.

I'm not sure how this could be resolved. Perhaps spec changes?

### Operating system

NixOS unstable

### Browser information

Firefox 97.0.1

### URL for webapp

develop.element.io

### Application version

Element version: 95de708f4ee7-react-1a6134e441ba-js-53aa34fba57c Olm version: 3.2.8

### Homeserver

Synapse 1.52.0

### Will you send logs?

No

Review the room-upgrade and space-child relationship in the relevant Matrix/Element specification, using the reported sequence as the starting case. Compare it with the room creation event and the twice-upgraded case; done means a newly invited user can identify the upgraded room as a child of the space.

Written by the indexing model from the issue text.