Validate `Environment.tier` presence always

Problem

We introduced background migration to backfill environments.tier in !105998 (merged). However, we don't validate the tier existence for all existing environments yet. We should validate for data integrity.

Proposal

• Make sure that the BackfillEnvironmentTiers BG migration completed.
• Cleanup the migration https://docs.gitlab.com/ee/development/database/batched_background_migrations.html#cleaning-up
  • Run finalize_batched_background_migration.
• Update the validation to validates :tier, presence: true in Environment model.

Auto Generated

The following discussion from !105998 (merged) should be addressed:

• @shinya.maeda started a discussion:

  I'll create a follow-up issue.