chore(control): Pending deletion for dead ExternalActorReplica table

[strongs]

second-to-last change to tear out the unused ExternalActorReplica table; move it to pending prior to actually dropping

The next PR in this series will be the final one, where the table actually gets dropped: PR link

[sentry]

Sentry Snapshot Testing

Name	Added	Removed	Changed	Renamed	Unchanged	Skipped	Status
sentry-frontend sentry-frontend	0	0	0	0	451	0	✅ Unchanged

⚙️ sentry-frontend Snapshot Settings

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/hybridcloud/migrations/0031_remove_externalactorreplica_pending.py

for 0031_remove_externalactorreplica_pending in hybridcloud

--
-- Alter field integration on externalactorreplica
--
SET CONSTRAINTS "hybridcloud_external_integration_id_f5d75f8b_fk_sentry_in" IMMEDIATE; ALTER TABLE "hybridcloud_externalactorreplica" DROP CONSTRAINT "hybridcloud_external_integration_id_f5d75f8b_fk_sentry_in";
--
-- Alter field user on externalactorreplica
--
SET CONSTRAINTS "hybridcloud_external_user_id_be164c6c_fk_auth_user" IMMEDIATE; ALTER TABLE "hybridcloud_externalactorreplica" DROP CONSTRAINT "hybridcloud_external_user_id_be164c6c_fk_auth_user";
--
-- Moved model ExternalActorReplica to pending deletion state
--
-- (no-op)