Fix rubocop config and warnings (#15503)

* disable NewCops

* update TargetRubyVersion

* Fix Lint/MissingSuper for ActiveModelSerializers::Model

* Fix Lint/MissingSuper for feed

* Fix Lint/FloatComparison

* Do not use instance variables

app/models/home_feed.rb

@@ -2,12 +2,11 @@
 
 class HomeFeed < Feed
   def initialize(account)
-    @type    = :home
-    @id      = account.id
     @account = account
+    super(:home, account.id)
   end
 
   def regenerating?
-    redis.exists?("account:#{@id}:regeneration")
+    redis.exists?("account:#{@account.id}:regeneration")
   end
 end