[CloudSecurity][Docs] Remove Defend for Containers (D4C) docs

[nick-alayil]

Summary

Remove all Defend for Containers (D4C) related documentation from Elastic Security documentation as part of the ongoing D4C deprecation process in Serverless and D4C deprecation process in ECH 9.0

Context

• D4C integration is being deprecated in Serverless
• This deprecation does not affect ECH documentation at this time
• ECH related changes will be addressed separately in 9.0.0 documentation

Required Changes

1. Remove the following documentation pages & sub-pages entirely:
   https://www.elastic.co/guide/en/serverless/current/security-d4c-overview.html

2. Update the billing documentation page:
   https://www.elastic.co/guide/en/serverless/current/security-billing.html#security-billing-cloud-protection

Remove the following references

• For [D4C](https://www.elastic.co/guide/en/serverless/current/security-d4c-overview.html), billing is based on how many agents (agent.id s) you use.

• and configuration drift prevention for Cloud Workloads.

• Add D4C breaking changes to 9.0 breaking changes and upgrade guide "If you have D4C in your 8.x deployment, make sure to remove it from all your Kubernetes clusters and agent policies before upgrade."

• In the 8.x docs D4C docs, add a warning that it is deprecated in 9.0.

[benironside]

fixed

[benironside]

keeping open to track ESS 9.0 requirement