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/command/instance_features_model_test.go

@@ -74,15 +74,6 @@ func Test_reduceInstanceFeature(t *testing.T) {
 			},
 			expected: &InstanceFeatures{OIDCSingleV1SessionTermination: gu.Ptr(true)},
 		},
-		{
-			name: "enable back channel logout",
-			args: args{
-				features: &InstanceFeatures{},
-				key:      feature.KeyEnableBackChannelLogout,
-				value:    true,
-			},
-			expected: &InstanceFeatures{EnableBackChannelLogout: gu.Ptr(true)},
-		},
 		{
 			name: "login v2",
 			args: args{