MigrateSharedVulnerabilityScannersV2 background migration

What does this MR do and why?

!89127 (merged) introduced a background migration for #357659 (closed):

Introduces a background migration that fixes vulnerability findings with erroneously associated vulnerability scanners.

Due to a bug, Vulnerabilities::Scanner records were erroneously reused across projects. This causes scanner: null GraphQL responses when querying project vulnerabilities due to lack of read permission.

Specifically, for concerned rows, the vulnerability_occurrences.project_id column mismatches the associated vulnerability_scanners.project_id.

However the migration applies its scope to sub-batches instead of batches. This results in a very long runtime (relevant Slack thread).

Here, a new verison of the migration is introduced, which applies its scope to batches. The difference to the currently running migration is essentially:

+ prepended do
+   scope_to -> (relation) { Finding.to_process.merge(relation) }
+ end

 def perform
   each_sub_batch(
     operation_name: :migrate_shared_vulnerability_scanners,
-    batching_scope: -> (relation) { Finding.to_process.merge(relation) }
  ) do |batch|
   # ...
 end

This MR:

• updates the original scheduling migration's #up and #down to no-ops
• updates the original background migration class MigrateSharedVulnerabilityScanners
• adds a new scheduling migration which in #up first uses delete_batched_background_migration and then queue_batched_background_migration to enqueue the updated background migration again

Migration output

Up

main: == 20220919080304 ScheduleMigrateSharedVulnerabilityScannersV2: migrating =====
main: == 20220919080304 ScheduleMigrateSharedVulnerabilityScannersV2: migrated (0.0554s) 

Down

main: == 20220919080304 ScheduleMigrateSharedVulnerabilityScannersV2: reverting =====
main: == 20220919080304 ScheduleMigrateSharedVulnerabilityScannersV2: reverted (0.0648s) 

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.