Fix /-/readiness probe for Puma Single

What does this MR do?

When running Puma in a Single mode (non-Clustered, workers=0),
the /readiness health check returned an error:
{"status":"failed","master_check":[{"status":"failed","message":"unexpected Master check result: false"}]}.

This MR fixes that – we simply don't run master_check if not in Clustered mode.

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

Start Puma in Single mode:

• open config/puma.rb
• set workers 0

Restart webserver

• on GDK: gdk restart rails-web

Run curl http://localhost:3000/-/readiness:

• On master, you will get error message: {"status":"failed","master_check":[{"status":"failed","message":"unexpected Master check result: false"}]}
• On this branch: {"status":"ok"}

Security

N/A

Related to #300661 (closed)