Update agent policy cpu limit

[Supplementing]

With the recent changes made in elastic/kibana#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

[Supplementing]

Blocked by https://github.com/elastic/ingest-dev/issues/2306. Waiting on agent feature to be complete and feature flag to be enabled before we need the docs updated

[Supplementing]

Apologies for the back and forth, just going to close for now.