How should m.room.canonical_alias be handled on room upgrade

**Link to problem area**: https://spec.matrix.org/v1.14/client-server-api/#server-behaviour-19

**Issue**

Step 4 of server behavior on the room upgrade endpoint is:

> Moves any local aliases to the new room.

The spec does not mention what should happen with `m.room.canonical_alias` explicitly. On synapse, it appears that local aliases are removed from `m.room.canonical_alias` on the old room, and added to `m.room.canonical_alias` on the new room.

Start with the room upgrade endpoint's server behavior in the linked Matrix client-server API section, then inspect how the specification describes m.room.canonical_alias and local aliases. Confirm the intended behavior for the old and new rooms, and update the relevant specification text so the canonical-alias outcome is explicit.

Written by the indexing model from the issue text.