Add rubocop rule to use Gitlab::Popen for popen3

What does this MR do?

Rubocop rule

Adds a rubocop rule to use Gitlab::Popen instead of popen3. Our Popen module contains methods that wrap Open3.popen3 in a way that avoids deadlocks.

Code change

Change one instance in our Rails app which was susceptible to a deadlock from using Open3.popen3 to Gitlab::Popen.popen_with_detail.

Documentation change

Developer documentation added for the Gitlab::Popen module methods.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ee/issues/28947

Does this MR meet the acceptance criteria?

• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides