docs: update roadmap in README

Permanent keys have been already implemented.

Author: josecelano

README.md

@@ -24,7 +24,6 @@
 Core:
 
 - [ ] New option `want_ip_from_query_string`. See <https://github.com/torrust/torrust-tracker/discussions/532#issuecomment-1836642956>.
-- [ ] Permanent keys. See <https://github.com/torrust/torrust-tracker/discussions/244>.
 - [ ] Peer and torrents specific statistics. See <https://github.com/torrust/torrust-tracker/discussions/139>.
 
 Persistence: