Migrate RedisHLL keys to default slot (backport)

Related to #280527 (closed)

master bug fix !115927 (merged)

Description from the fix:

It was discovered that in !110952 (merged) there was a bug in Redis data migration script.

The data wasn't migrated because https://gitlab.com/gitlab-> org/gitlab/-/merge_requests/110952/diffs#a7f91c8c1fa7a01083a46f29f24893a18168a137_0_72 wasn't actually loading events (called from the wrong >place), events aren't stored in db/post_migration folder.

This MR re-introduces the migration with the bug being fixed.

At the moment we have data being added to Redis with new keys. Old keys are still in Redis. ServicePing aggregates data taking the new keys into account.

This MR merged old keys with new ones, so we don't loose the old data on Gitlab.com

Custom redis_slot will be removed a following MR, it's not in use in the current codebase but still required for migration.

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).
• Ensure the e2e:package-and-test job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

The process of backporting bug fixes into stable branches is tracked as part of an internal pilot. If you have questions about this process, please:

• Refer to the internal pilot issue for feedback or questions.
• Refer to the patch release runbook for engineers and maintainers for guidance.