Fix pages migration threads

What does this MR do?

From rails docs:

Each thread should be wrapped before it runs application code, so if your application manually delegates work to other threads, such as via Thread.new or Concurrent Ruby features that use thread pools, you should immediately wrap the block:

Thread.new do
  Rails.application.executor.wrap do
    # your code here
  end
end

We didn't do that, and we think it caused #300003 (closed)

Screenshots (strongly suggested)

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #300003 (closed)