[DOCS] [META] Rename integration to "Elastic Defend"

[joepeeples]

Description

Per https://github.com/elastic/security-team/issues/4759, we are renaming the Endpoint and Cloud Security integration to Elastic Defend.

Component docs issues and tasks:

(Note that some issues require updating content in other repos besides elastic/security-docs.)

• Create new attribute for "Elastic Defend" docs#2516
• [DOCS] Update attributes and text references for Elastic Defend integration #2464
• [DOCS] Update screenshots of Elastic Defend #2465
• Update references in Observability/Fleet docs (https://github.com/elastic/observability-docs/issues/2326)
• Update references in Getting Started pages (SIEM and endpoint) (elastic/tech-content) (these pages are getting revamped soon; no need to update)
• [DOCS] Guided Onboarding for Elastic Defend #2457 -- AWP team is developing it, but onboarding is for both cloud and endpoint workflows
• Clarify language: explicit scenarios of when to use Elastic Defend, Elastic Agent, Elastic Endpoint, etc.

Related issues/PRs:

• Main dev issue for Security Team:
  https://github.com/elastic/security-team/issues/4759

• Onboarding - Cloud:
  https://github.com/elastic/security-team/issues/3981

• Reference - previous renaming (8.3)
  META: Rename Endpoint Security to Endpoint and Cloud Security #2025

[joepeeples]

All tasks completed/resolved.