Add new variable forgotten_room_retention_period

[felixx9]

Add variable to purge empty rooms (new feature in latest release).

I haven't found, that it's implemented. Maybe I'm wrong!?

• Add automatic purge after all users forget a room: Add automatic purge after all users forget a room matrix-org/synapse#15488
• need to be activated by setting a variable called forgotten_room_retention_period: Add automatic purge after all users forget a room matrix-org/synapse#15488 (comment)