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.

config/unicorn.conf.rb

@@ -64,15 +64,7 @@ initialized = false
 before_fork do |server, worker|
   unless initialized
     Discourse.preload_rails!
-
-    # V8 does not support forking, make sure all contexts are disposed
-    ObjectSpace.each_object(MiniRacer::Context) { |c| c.dispose }
-
-    # get rid of rubbish so we don't share it
-    # longer term we will use compact! here
-    GC.start
-    GC.start
-    GC.start
+    Discourse.before_fork
 
     initialized = true