feat: fingerprint_sha256 for SSH keys background migration

What does this MR do?

This is a follow-up MR of !19860 (merged) we decided here !19860 (comment 257781711) to postpone the background migration, so here it is.

Related to #39455 (closed)

Estimated time

There are around 3 millions records on keys table for GitLab.com. Each background worker will update batches of 1_000 with a 5.minute interval, which gives around 10 days of execution time.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers