Promote FF_RETRIEVE_POD_WARNING_EVENTS to a config print_pod_warning_events

What does this MR do?

Promote FF_RETRIEVE_POD_WARNING_EVENTS to a config print_pod_warning_events

Why was this MR needed?

FF_RETRIEVE_POD_WARNING_EVENTS is being deprecated in %18.0 and there is no alternative to turn this off users might have no way to stop these logs.

After investigations, it also looks like the feature flag was mistakenly removed in this MR: Always attempt to retrieve pod warning events (!4852 - merged) 😞

What's the best way to test this MR?

❯ go test -tags integration,kubernetes -timeout=60m -run "^TestRunIntegrationTestsWithFeatureFlag/testKubernetesClusterWarningEvent*" -v gitlab.com/gitlab-org/gitlab-runner/executors/kubernetes

What are the relevant issue numbers?

close #38479 (closed) gitlab-org/charts/gitlab-runner#499 (closed)