DEV: Upgrade Rails to 6.1.3.1 (#12688)

Rails 6.1.3.1 deprecates a few API and has some internal changes that break our tests suite, so this commit fixes all the deprecations and errors and now Discourse should be fully compatible with Rails 6.1.3.1. We also have a new release of the rails_failover gem that's compatible with Rails 6.1.3.1.

lib/validators/unique_among_validator.rb

@@ -2,21 +2,36 @@
 
 class UniqueAmongValidator < ActiveRecord::Validations::UniquenessValidator
   def validate_each(record, attribute, value)
-    old_errors = record.errors[attribute].size
+    old_errors = []
+    record.errors.each do |error|
+      old_errors << error if error.attribute == attribute
+    end
 
     # look for any duplicates at all
     super
 
-    new_errors = record.errors[attribute].size - old_errors
+    new_errors = []
+    record.errors.each do |error|
+      new_errors << error if error.attribute == attribute
+    end
 
     # do nothing further unless there were some duplicates.
-    unless new_errors == 0
+    if new_errors.size - old_errors.size != 0
       # now look only in the collection we care about.
       dupes = options[:collection].call(record).where("lower(#{attribute}) = ?", value.downcase)
       dupes = dupes.where("id != ?", record.id) if record.persisted?
 
       # pop off the error, if it was a false positive
-      record.errors[attribute].pop(new_errors) unless dupes.exists?
+      if !dupes.exists?
+        record.errors.delete(attribute)
+        old_errors.each do |error|
+          record.errors.add(
+            error.attribute,
+            error.type,
+            **error.options
+          )
+        end
+      end
     end
   end