Set the default gitlab-shell timeout to 3 hours

What does this MR do?

Increases the default timeout on gitlab-shell operations from 800 seconds to 3 hours.

This should cause all sidekiq jobs waiting on a subprocess to fail, resulting in the job being re-enqueued.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Without this, large Git repositories will never import successfully. It also affects Geo repository synchronization.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39977 (closed)