Update the Sidekiq worker removal instructions

🔬 What does this MR do?

In https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#in-the-first-minor-release, there are some instructions that are confusing. Step 4 states:

In a separate merge request, because it’s a migration, consider using the sidekiq_remove_jobs helper migration method in a post-deployment migration:

I don't see why it has to go in a different MR.

Actually, it is better if the sidekiq_remove_jobs migration goes with MR 2 (https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#in-a-subsequent-separate-minor-release).

Why?

• The first MR still have the worker class (but it's a no-op). We could still miss code that enqueues the jobs (step 1). This means that even if we have the migration there, we could still have code that enqueues the job.
• On the second MR, we remove the class itself. This will catch any leftover code that enqueues the job (because we need the class to enqueue the job). The CI pipeline will certainly be ❌ if we try to read a class that doesn't exist anymore. Once this MR is deployed, it's a good idea to run the sidekiq_remove_jobs migration.

⛓ Related issues

None

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