Make db:migrate:reset work with GITLAB_USE_MODEL_LOAD_BALANCING=true

The problem was caused by using the the ActsAsTaggableOn::Tagging model
directly which has some weird configuration in
https://gitlab.com/gitlab-org/gitlab/-/blob/9c2682cf013dae1d469e6c2c6fea3f6d1aa0fa5a/config/initializers/1_acts_as_taggable.rb#L15
which forces the connection to use the CI connection. But this doesn't
make sense in the migration context where we are migrating the schema
for the `main` database. As such we need to just define a model in the
migration like we would normally do then it will use the correct
connection for the current migration.

.gitlab/ci/rails.gitlab-ci.yml

@@ -377,8 +377,6 @@ db:migrate:reset decomposed:
     - db:migrate:reset
     - .decomposed-database
     - .rails:rules:decomposed-databases
-  variables:
-    GITLAB_USE_MODEL_LOAD_BALANCING: "false"
 
 db:migrate-from-previous-major-version:
   extends: .db-job-base

db/post_migrate/20211028100843_delete_issue_merge_request_taggings_records.rb

@@ -8,8 +8,12 @@ class DeleteIssueMergeRequestTaggingsRecords < Gitlab::Database::Migration[1.0]
   BATCH_SIZE = 3_000
   TAGGABLE_TYPES = %w(Issue MergeRequest)
 
+  class Tagging < ActiveRecord::Base
+    self.table_name = "taggings"
+  end
+
   def up
-    sleep 2 while ActsAsTaggableOn::Tagging.where(taggable_type: TAGGABLE_TYPES).limit(BATCH_SIZE).delete_all > 0
+    sleep 2 while Tagging.where(taggable_type: TAGGABLE_TYPES).limit(BATCH_SIZE).delete_all > 0
 
     remove_concurrent_index_by_name :taggings, INDEX_NAME
   end