[mod] limiter: block unmaintained Farside instances

Since [bb3a01f8] has been merged to the Farside project, Farside instances do no
longer need to send requests to SearXNG instances [1].

There are some old unmaintained Farside instances on the web that continue to
query SearXNG instances --> we can safely block their requests.

[1] https://github.com/benbusby/farside/issues/95
[bb3a01f8] https://github.com/benbusby/farside/commit/bb3a01f8



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>