Fix to remove foreign key for geo_event_log

What does this MR do and why?

Fix migration db/post_migrate/20240425170527_remove_foreign_keys_geo_event_log.rb as it's breaking the db testing, e.g. https://ops.gitlab.net/gitlab-com/database-team/gitlab-com-database-testing/-/jobs/13813871

If we check the foreign key name in ci database, the foreign key name is actually fk_rails_78a6492f68 instead of fk_78a6492f68 referencing table geo_repository_updated_events , see https://console.postgres.ai/gitlab/gitlab-production-ci/sessions/28078/commands/87450

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Related to #439935 (closed)