feat(oidc): move back-channel logout from beta to GA (#11493)

# Which Problems Are Solved

This PR marks the OIDC Back-Channel Logout as general available (GA).

# How the Problems Are Solved

- API:
- deprecated the feature toggle (to prevent breaking changes) in v2beta
and v2 API
  - removed all related event logic and updated the projection iteration
  - removed the toggle usage for back-channel logout
  - removed related translations
- Management Console:
  - removed the feature toggle and its translations
- Docs:
  - removed all beta labels and feature toggle notes

# Additional Changes

none

# Additional Context

- closes #11277 
- requires backport to v4.x

console/src/app/components/features/features.component.ts

@@ -29,7 +29,6 @@ import { LoginV2FeatureToggleComponent } from '../feature-toggle/login-v2-featur
 const FEATURE_KEYS = [
   'consoleUseV2UserApi',
   'debugOidcParentError',
-  'enableBackChannelLogout',
   // 'improvedPerformance',
   'loginDefaultOrg',
   'oidcSingleV1SessionTermination',