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.go

@@ -18,7 +18,6 @@ type InstanceFeatures struct {
 	ImprovedPerformance            []feature.ImprovedPerformanceType
 	DebugOIDCParentError           *bool
 	OIDCSingleV1SessionTermination *bool
-	EnableBackChannelLogout        *bool
 	LoginV2                        *feature.LoginV2
 	PermissionCheckV2              *bool
 	ManagementConsoleUseV2UserApi  *bool
@@ -32,7 +31,6 @@ func (m *InstanceFeatures) isEmpty() bool {
 		m.ImprovedPerformance == nil &&
 		m.DebugOIDCParentError == nil &&
 		m.OIDCSingleV1SessionTermination == nil &&
-		m.EnableBackChannelLogout == nil &&
 		m.LoginV2 == nil &&
 		m.PermissionCheckV2 == nil &&
 		m.ManagementConsoleUseV2UserApi == nil &&