Update agent policy cpu limit

With the recent changes made in https://github.com/elastic/kibana/pull/219772, the docs at https://www.elastic.co/docs/reference/fleet/agent-policy#agent-policy-limit-cpu that outline setting the maximum CPU usage need to be updated. Primarily, we are no longer limiting the process count, but instead, allowing the user to set a percentage of the CPU that agents belonging to the policy can use. This no longer relies on the `max_procs` field at all, so it can be removed. Please see the video in the PR for more context and reach out for more information! Point of contact: @Supplementing Stakeholder(s): @elastic/fleet