Add foreign key for ci_stages.pipeline_id_convert_to_bigint

What does this MR do and why?

This is to prepare the foreign key for ci_stages.pipeline_id_convert_to_bigint

• Create invalid foreign key in a regular migration. See https://docs.gitlab.com/ee/development/database/add_foreign_key_to_existing_column.html
• Prepare the async foreign key validation in post migration. See https://docs.gitlab.com/ee/development/database/add_foreign_key_to_existing_column.html#schedule-the-fk-to-be-validated

The required indexes are created, see the following output from postgres.ai:

Table "public.ci_stages"
            Column             |            Type             | Collation | Nullable |                Default                | Storage  | Compression | Stats target | Description 
-------------------------------+-----------------------------+-----------+----------+---------------------------------------+----------+-------------+--------------+-------------
 project_id                    | integer                     |           |          |                                       | plain    |             |              | 
 pipeline_id                   | integer                     |           |          |                                       | plain    |             |              | 
 created_at                    | timestamp without time zone |           |          |                                       | plain    |             |              | 
 updated_at                    | timestamp without time zone |           |          |                                       | plain    |             |              | 
 name                          | character varying           |           |          |                                       | extended |             |              | 
 status                        | integer                     |           |          |                                       | plain    |             |              | 
 lock_version                  | integer                     |           |          | 0                                     | plain    |             |              | 
 position                      | integer                     |           |          |                                       | plain    |             |              | 
 id                            | bigint                      |           | not null | nextval('ci_stages_id_seq'::regclass) | plain    |             |              | 
 partition_id                  | bigint                      |           | not null |                                       | plain    |             |              | 
 pipeline_id_convert_to_bigint | bigint                      |           |          |                                       | plain    |             |              | 
Indexes:
    "ci_stages_pkey" PRIMARY KEY, btree (id)
    "index_ci_stages_on_pipeline_id" btree (pipeline_id)
    "index_ci_stages_on_pipeline_id_and_id" btree (pipeline_id, id) WHERE status = ANY (ARRAY[0, 1, 2, 8, 9, 10])
    "index_ci_stages_on_pipeline_id_and_name" UNIQUE, btree (pipeline_id, name)
    "index_ci_stages_on_pipeline_id_and_position" btree (pipeline_id, "position")
    "index_ci_stages_on_pipeline_id_convert_to_bigint" btree (pipeline_id_convert_to_bigint)
    "index_ci_stages_on_pipeline_id_convert_to_bigint_and_id" btree (pipeline_id_convert_to_bigint, id) WHERE status = ANY (ARRAY[0, 1, 2, 8, 9, 10])
    "index_ci_stages_on_pipeline_id_convert_to_bigint_and_name" UNIQUE, btree (pipeline_id_convert_to_bigint, name)
    "index_ci_stages_on_pipeline_id_convert_to_bigint_and_position" btree (pipeline_id_convert_to_bigint, "position")
    "index_ci_stages_on_project_id" btree (project_id)
Check constraints:
    "check_81b431e49b" CHECK (lock_version IS NOT NULL)
Foreign-key constraints:
    "fk_fb57e6cc56" FOREIGN KEY (pipeline_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE
Referenced by:
    TABLE "p_ci_builds" CONSTRAINT "fk_3a9eaa254d" FOREIGN KEY (stage_id) REFERENCES ci_stages(id) ON DELETE CASCADE
Triggers:
    trigger_07bc3c48f407 BEFORE INSERT OR UPDATE ON ci_stages FOR EACH ROW EXECUTE FUNCTION trigger_07bc3c48f407()
Access method: heap

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How to set up and validate locally

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Related to #422814 (closed)