Resolve "Project is being serialized when enqueueing to sidekiq"

What does this MR do?

Todos::Destroy::EntityLeaveService#project_ids was returning ActiveRecord objects with IDs, not simply IDs. That means we were serialising more than we needed to in Sidekiq.

We can simply rename this method to #projects as this class doesn't use any of the superclass methods that would use #project_ids.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes #59265 (closed).