Fix endpoint urgency

What does this MR do and why?

The urgency level for Apdex SLI can't be adjusted in a API class by putting the directive on a namespace.

Example

kubernetes/agent_configuration API endpoint has urgency: :low (5s) defined in it's namespace:

namespace 'kubernetes/agent_configuration', urgency: :low do

Yet the logs show the default value is used (1s):

Same for modules/starboard_vulnerability

Solution

This MR moves the urgency definition to each specific endpoints, and adds a warning in the documentation.