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

internal/query/instance_features_model.go

@@ -67,7 +67,6 @@ func (m *InstanceFeaturesReadModel) Query() *eventstore.SearchQueryBuilder {
 			feature_v2.InstanceImprovedPerformanceEventType,
 			feature_v2.InstanceDebugOIDCParentErrorEventType,
 			feature_v2.InstanceOIDCSingleV1SessionTerminationEventType,
-			feature_v2.InstanceEnableBackChannelLogout,
 			feature_v2.InstanceLoginVersion,
 			feature_v2.InstancePermissionCheckV2,
 			feature_v2.InstanceManagementConsoleUseV2UserApi,
@@ -93,7 +92,6 @@ func (m *InstanceFeaturesReadModel) populateFromSystem() bool {
 	m.instance.UserSchema = m.system.UserSchema
 	m.instance.ImprovedPerformance = m.system.ImprovedPerformance
 	m.instance.OIDCSingleV1SessionTermination = m.system.OIDCSingleV1SessionTermination
-	m.instance.EnableBackChannelLogout = m.system.EnableBackChannelLogout
 	m.instance.LoginV2 = m.system.LoginV2
 	return true
 }
@@ -116,8 +114,6 @@ func reduceInstanceFeatureSet[T any](features *InstanceFeatures, event *feature_
 		features.DebugOIDCParentError.set(level, event.Value)
 	case feature.KeyOIDCSingleV1SessionTermination:
 		features.OIDCSingleV1SessionTermination.set(level, event.Value)
-	case feature.KeyEnableBackChannelLogout:
-		features.EnableBackChannelLogout.set(level, event.Value)
 	case feature.KeyLoginV2:
 		features.LoginV2.set(level, event.Value)
 	case feature.KeyPermissionCheckV2: