Fix cross-database modification detection with HasAndBelongsToMany

The JOIN-table for ActiveRecord associations using has_and_belongs_to_many has no model file so there is no association with the gitlab_schema. This makes the cross-database modification detection raise errors with an undefined table.

Example:

Database::PreventCrossDatabaseModification::CrossDatabaseModificationAcrossUnsupportedTablesError:
       Cross-database data modification of 'gitlab_main, undefined' were detected within a transaction modifying the 'customer_relations_contacts, issue_customer_relations_contacts' tables

MR where the error was raised: !71889 (merged)

Option 1: Turn has_and_belongs_to_many associations to has_many through associations

We use has_and_belongs_to_many in the following files (ag -l has_and_belongs_to_many):

app/models/issue.rb
app/models/customer_relations/contact.rb
app/models/prometheus_alert_event.rb
app/models/self_managed_prometheus_alert_event.rb
ee/app/models/approval_project_rule.rb
ee/app/models/merge_requests/external_status_check.rb
ee/app/models/approval_merge_request_rule.rb
ee/app/models/concerns/approval_rule_like.rb
ee/app/models/concerns/ee/protected_branch.rb
ee/lib/ee/gitlab/background_migration/migrate_approver_to_approval_rules.rb
lib/gitlab/background_migration/add_modified_to_approval_merge_request_rule.rb

How:

1. Define ActiveRecord models for the defined join_tables.
2. Update has_and_belongs_to_many to has_many through:

If we fix this issue then we can definitely remove a few entries from the spec/support/database/cross-database-modification-allowlist.yml allowlist file.