FIX: Ensure we dispose of MiniRacer::Context before forking daemons (#28361)

This commit updates `Demon::Base#start` to call `Discourse.before_fork`
before forking. According to the docs in `mini_racer`, we need to
"Dispose manually of all MiniRacer::Context objects prior to forking".

This commit is motivated by a segmentation fault which we are seeing in
production when killing a daemon process. Backtrace of the core dump
includes traces of `mini_racer` so we think this is the cause. Note that
we are not 100% sure if this will fix the issue.

lib/demon/base.rb

@@ -154,6 +154,8 @@ class Demon::Base
   end
 
   def run
+    Discourse.before_fork if defined?(Discourse)
+
     @pid =
       fork do
         Process.setproctitle("discourse #{self.class.prefix}")
@@ -161,6 +163,7 @@ class Demon::Base
         establish_app
         after_fork
       end
+
     write_pid_file
   end