Slowly iterate MigrateSharedVulnerabilityIdentifiers

What does this MR do and why?

%15.11 backport of !121377 (merged)

From the Backporting Request (https://gitlab.com/gitlab-org/release/tasks/-/issues/5777):

• %15.11 released the MigrateSharedVulnerabilityIdentifiers background migration (!112556 (merged)).
• The migration corrects inconsistent data in the vulnerability_occurrences table that resulted from an API bug.
• The migration contains a bug that can keep the migration from completing.
• The bug triggered in production, and the migration appeared to be stuck at 0% in active state.
• If the bug triggers, the migration still corrects all affected rows, but fails to complete.
• If the migration gets stuck, it has to be paused manually, so that other migrations targeting the same table can execute.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).

Related to #359963 (closed)