Initialize bigint conversion for vulnerability_occurrence_pipelines

What does this MR do and why?

Initialize bigint conversion for vulnerability_occurrence_pipelines

This is the first step to convert pipeline_id related to bigint for:

• vulnerability_occurrence_pipelines.pipeline_id

See https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#initialize-the-conversion-and-start-migrating-existing-data-release-n

Estimation for different params

Sub batch size	Batch size	Pause(ms)	Max avg query(ms)	Estimation	Link
100			1094.2	Sampled 35 batches. Estimated Time to complete: 5 months, 1 week, 5 days, 12 hours, 50 minutes, and 30 seconds	!148735 (comment 1850120097)
750	75_000	0	388.8	Sampled 37 batches. Estimated Time to complete: 1 month, 1 week, 6 days, 11 hours, 42 minutes, and 54 seconds	!148735 (comment 1850175526)
2000	75_000	0	1030.8	Sampled 42 batches. Estimated Time to complete: 1 month, 1 week, 6 days, 11 hours, 42 minutes, and 54 seconds	!148735 (comment 1850205574)
500	200_000	0	465.8	Sampled 8 batches. Estimated Time to complete: 1 month, 1 day, 15 hours, 57 minutes, and 46 seconds	!148735 (comment 1850391447)
500	100_000	0	569.0	Sampled 14 batches. Estimated Time to complete: 1 month, 6 days, 6 hours, 20 minutes, and 47 seconds	!148735 (comment 1850439710)

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Related to #454419 (closed)