MSC4164: Leave all rooms on deactivation

While most servers already do this, I think it would be good to actually recommend this in the spec directly, so that future servers know to implement this as well.

Rendered

Implementations:

Synapse: https://github.com/matrix-org/synapse/pull/3201
Dendrite: https://github.com/matrix-org/dendrite/pull/2545
Conduit: https://gitlab.com/famedly/conduit/-/commit/b81939841b272206991041ce7643a1c572590db3

matrix-org / matrix-spec-proposals

MSC4164: Leave all rooms on deactivation #4164