chore(oidc): graduate webkey to stable (#10122)

# Which Problems Are Solved

Stabilize the usage of webkeys.

# How the Problems Are Solved

- Remove all legacy signing key code from the OIDC API
- Remove the webkey feature flag from proto
- Remove the webkey feature flag from console
- Cleanup documentation

# Additional Changes

- Resolved some canonical header linter errors in OIDC
- Use the constant for `projections.lock` in the saml package.

# Additional Context

- Closes #10029
- After #10105
- After #10061

internal/query/instance_features_model.go

@@ -67,7 +67,6 @@ func (m *InstanceFeaturesReadModel) Query() *eventstore.SearchQueryBuilder {
 			feature_v2.InstanceUserSchemaEventType,
 			feature_v2.InstanceTokenExchangeEventType,
 			feature_v2.InstanceImprovedPerformanceEventType,
-			feature_v2.InstanceWebKeyEventType,
 			feature_v2.InstanceDebugOIDCParentErrorEventType,
 			feature_v2.InstanceOIDCSingleV1SessionTerminationEventType,
 			feature_v2.InstanceDisableUserTokenEvent,
@@ -121,8 +120,6 @@ func reduceInstanceFeatureSet[T any](features *InstanceFeatures, event *feature_
 		features.TokenExchange.set(level, event.Value)
 	case feature.KeyImprovedPerformance:
 		features.ImprovedPerformance.set(level, event.Value)
-	case feature.KeyWebKey:
-		features.WebKey.set(level, event.Value)
 	case feature.KeyDebugOIDCParentError:
 		features.DebugOIDCParentError.set(level, event.Value)
 	case feature.KeyOIDCSingleV1SessionTermination: