Introduce limit to number of registered runners [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR is a follow-up to !60157 (merged) that implements more user-friendly messages in case the runner registration limits are hit.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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Security

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