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/projection/instance_features.go

@@ -14,7 +14,7 @@ import (
 )
 
 const (
-	InstanceFeatureTable = "projections.instance_features4"
+	InstanceFeatureTable = "projections.instance_features5"
 
 	InstanceFeatureInstanceIDCol   = "instance_id"
 	InstanceFeatureKeyCol          = "key"
@@ -80,10 +80,6 @@ func (*instanceFeatureProjection) Reducers() []handler.AggregateReducer {
 				Event:  feature_v2.InstanceOIDCSingleV1SessionTerminationEventType,
 				Reduce: reduceInstanceSetFeature[bool],
 			},
-			{
-				Event:  feature_v2.InstanceEnableBackChannelLogout,
-				Reduce: reduceInstanceSetFeature[bool],
-			},
 			{
 				Event:  feature_v2.InstanceLoginVersion,
 				Reduce: reduceInstanceSetFeature[*feature.LoginV2],