Reverts forget_rooms_on_leave to upstream as this is too dangerous to change in a breaking way like this is doing. Especially doing it together with a Call to Action to update your Synapses causing this change to enter all playbook users playbooks who update.

Its safe to keep the version bump part of #2698 but not safe to keep this change that this commit is undoing by setting the value to false. This means our config now includes the value like a upstream config would but we keep it at false just like upstream does.