Do not use dedicated gitlab.rails.puma.single_mode

What does this Merge Request do?

Following our discussions in !1807 (merged), we drop dedicated single_mode config to be in line with Puma configuration (setting workers 0 should be enough).

Depends on gitlab!53830 (merged)

How to test locally

Switch to this branch in GDK

Try various configs in gdk.yml, run gdk reconfigure after each one, check the console output (that workers is updated and min/max threads as well depending on the values)

---
gitlab:
  rails:
    puma:
      workers: 0

Single mode - expect 4/4 threads min/max

---
gitlab:
  rails:
    puma:
      workers: 3

Expect 3 workers, 1/4 threads min/max threads (default)

---
gitlab:
  rails:
    puma:

Expect 2 workers (default), 1/4 min/max threads (default)

---
gitlab:
  rails:
    puma:
      workers: 0
      threads_max: 2

Single mode - expect 2/2 threads min/max (min is set to max because Single)

5. Funky example

---
gitlab:
  rails:
    puma:
      workers: 3
      threads_max: 2
      threads_min: 3

Puma will raise an issue: RuntimeError: The minimum (3) number of threads must be less than or equal to the max (2)

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.