Ensure that migration specs work without bot_type

Before this commit some migration specs failed with

    4) Gitlab::BackgroundMigration::DeleteDiffFiles#perform when diff
       files can be deleted rollsback if something goes wrong
       Failure/Error: return false if internal?

       NameError:
         undefined local variable or method `bot_type' for
           #<User id: @namespace1117>
         Did you mean?  bot_type=
                        bot_type?
       # ./ee/app/models/ee/user.rb:281:in `bot?'
       # ./ee/app/models/ee/user.rb:277:in `internal?'
       # ./app/models/user.rb:1452:in `password_required?'
       # ./ee/app/models/ee/user.rb:290:in `password_required?'

https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/185393284

It's because we migrate down before each migration spec which actually
introduces `bot_type`.

ee/app/models/ee/user.rb

@@ -242,7 +242,7 @@ module EE
     end
 
     def bot?
-      bot_type.present?
+      has_bot_type? ? bot_type.present? : old_support_bot
     end
 
     protected
@@ -256,6 +256,14 @@ module EE
 
     private
 
+    def has_bot_type?
+      has_attribute?(:bot_type)
+    end
+
+    def old_support_bot
+      read_attribute(:support_bot)
+    end
+
     def namespace_union(select = :id)
       ::Gitlab::SQL::Union.new([
         ::Namespace.select(select).where(type: nil, owner: self),