Draft: Dropping table external_approval_rules_protected_branches

What does this MR do and why?

As part of #442746 (closed), dropping table external_approval_rules_protected_branches as it is not being used anywhere and a new table external_status_checks_protected_branches was created in place of it in !62186 (merged).

I think it got forgotten to get deleted at that time. As you can see in https://console.postgres.ai/gitlab/gitlab-production-main/sessions/27416/commands/85384, there are no rows in this table.

Database

up

bundle exec rake db:migrate:up:main VERSION=20240327102015
main: == [advisory_lock_connection] object_id: 120620, pg_backend_pid: 65177
main: == 20240327102015 DropExternalApprovalRulesProtectedBranches: migrating =======
main: -- drop_table(:external_approval_rules_protected_branches, {:if_exists=>true})
main:    -> 0.0055s
main: == 20240327102015 DropExternalApprovalRulesProtectedBranches: migrated (0.0088s) 

main: == [advisory_lock_connection] object_id: 120620, pg_backend_pid: 65177

down

bundle exec rake db:migrate:down:main VERSION=20240327102015
main: == [advisory_lock_connection] object_id: 120620, pg_backend_pid: 66395
main: == 20240327102015 DropExternalApprovalRulesProtectedBranches: reverting =======
main: -- create_table(:external_approval_rules_protected_branches)
main:    -> 0.0041s
main: == 20240327102015 DropExternalApprovalRulesProtectedBranches: reverted (0.0168s) 

main: == [advisory_lock_connection] object_id: 120620, pg_backend_pid: 66395

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Related to #442746 (closed)