Update m.id.phone to use 'phone' instead of 'number' (#7687)

The spec [states](https://matrix.org/docs/spec/client_server/r0.6.1#phone-number) that `m.id.phone` requires the field `country` and `phone`.

In Synapse, we've been enforcing `country` and `number`.

I am not currently sure whether this affects any client implementations.

This issue was introduced in #1994.