Make db:migrate:reset work with GITLAB_USE_MODEL_LOAD_BALANCING=true

What does this MR do and why?

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.

This fixes the following error:

PG::UndefinedTable: ERROR:  relation "taggings" does not exist
LINE 8:  WHERE a.attrelid = '"taggings"'::regclass

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Related to #355573 (closed)