Promote the FF_RETRIEVE_POD_WARNING_EVENTS feature flag to a setting

As per #38402 (comment 2297419639), we are planning to remove the FF_RETRIEVE_POD_WARNING_EVENTS in 18.0 but with no alternative to turn this off users might have no way to stop these logs.

Let's promote the FF to a setting now.

[runners.kubernetes]
print_pod_warning_events = false (true by default)

Or it could be

[runners.kubernetes]
retrieve_pod_warning_events = false (true by default)

I would vote for 1. since we are printing the events and retrieve implies we are doing something more internal with them.