Synchronously validate `finding_id` FK on vulnerabilities

What does this MR do and why?

The foreign key was validated asynchronously in MR 131979

This commit is the async followup per the documentation

resolves: #425409 (closed)

---

Changelog: changed
EE: true

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Screenshots or screen recordings

verifying async finished

1. on postgres.ai, running \d+ vulnerabilities indicates the FK is valid:
2. The MR widget from async MR shows Deployed to db/‎gprd‎ Dec 28, 2023, 3:55 AM
3. chatops also shows it on db/gprd

MR widget from async migration	\d+ vulnerabilities	/chatops run auto_deploy status 518dc47f7f11ec980ec7acf893b22c83520d0c42

migration output

bundle exec rails db:migrate

main: == [advisory_lock_connection] object_id: 182880, pg_backend_pid: 361338
main: == 20240102101409 ValidateFindingIdOnVulnerabilities: migrating ===============
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- execute("ALTER TABLE vulnerabilities VALIDATE CONSTRAINT fk_4e64972902;")
main:    -> 0.0004s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20240102101409 ValidateFindingIdOnVulnerabilities: migrated (0.0275s) ======

main: == [advisory_lock_connection] object_id: 182880, pg_backend_pid: 361338
ci: == [advisory_lock_connection] object_id: 183120, pg_backend_pid: 361340
ci: == 20240102101409 ValidateFindingIdOnVulnerabilities: migrating ===============
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- execute("ALTER TABLE vulnerabilities VALIDATE CONSTRAINT fk_4e64972902;")
ci:    -> 0.0004s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20240102101409 ValidateFindingIdOnVulnerabilities: migrated (0.0153s) ======

ci: == [advisory_lock_connection] object_id: 183120, pg_backend_pid: 361340

bundle exec rails db:rollback:main

main: == [advisory_lock_connection] object_id: 182360, pg_backend_pid: 360456
main: == 20240102101409 ValidateFindingIdOnVulnerabilities: reverting ===============
main: == 20240102101409 ValidateFindingIdOnVulnerabilities: reverted (0.0031s) ======

main: == [advisory_lock_connection] object_id: 182360, pg_backend_pid: 360456

bundle exec rails db:rollback:ci

ci: == [advisory_lock_connection] object_id: 182320, pg_backend_pid: 360889
ci: == 20240102101409 ValidateFindingIdOnVulnerabilities: reverting ===============
ci: == 20240102101409 ValidateFindingIdOnVulnerabilities: reverted (0.0086s) ======

ci: == [advisory_lock_connection] object_id: 182320, pg_backend_pid: 360889

Related to #425409 (closed)