chore(oidc): remove legacy storage methods (#10061)

# Which Problems Are Solved

Stabilize the optimized introspection code and cleanup unused code.

# How the Problems Are Solved

- `oidc_legacy_introspection` feature flag is removed and reserved.
- `OPStorage` which are no longer needed have their bodies removed.
- The method definitions need to remain in place so the interface
remains implemented.
  - A panic is thrown in case any such method is still called

# Additional Changes

- A number of `OPStorage` methods related to token creation were already
unused. These are also cleaned up.

# Additional Context

- Closes #10027 
- #7822

---------

Co-authored-by: Livio Spring <livio.a@gmail.com>

internal/query/instance_features_model.go

@@ -64,7 +64,6 @@ func (m *InstanceFeaturesReadModel) Query() *eventstore.SearchQueryBuilder {
 			feature_v2.InstanceResetEventType,
 			feature_v2.InstanceLoginDefaultOrgEventType,
 			feature_v2.InstanceTriggerIntrospectionProjectionsEventType,
-			feature_v2.InstanceLegacyIntrospectionEventType,
 			feature_v2.InstanceUserSchemaEventType,
 			feature_v2.InstanceTokenExchangeEventType,
 			feature_v2.InstanceImprovedPerformanceEventType,
@@ -94,7 +93,6 @@ func (m *InstanceFeaturesReadModel) populateFromSystem() bool {
 	}
 	m.instance.LoginDefaultOrg = m.system.LoginDefaultOrg
 	m.instance.TriggerIntrospectionProjections = m.system.TriggerIntrospectionProjections
-	m.instance.LegacyIntrospection = m.system.LegacyIntrospection
 	m.instance.UserSchema = m.system.UserSchema
 	m.instance.TokenExchange = m.system.TokenExchange
 	m.instance.ImprovedPerformance = m.system.ImprovedPerformance
@@ -111,15 +109,12 @@ func reduceInstanceFeatureSet[T any](features *InstanceFeatures, event *feature_
 		return err
 	}
 	switch key {
-	case feature.KeyUnspecified,
-		feature.KeyActionsDeprecated:
+	case feature.KeyUnspecified:
 		return nil
 	case feature.KeyLoginDefaultOrg:
 		features.LoginDefaultOrg.set(level, event.Value)
 	case feature.KeyTriggerIntrospectionProjections:
 		features.TriggerIntrospectionProjections.set(level, event.Value)
-	case feature.KeyLegacyIntrospection:
-		features.LegacyIntrospection.set(level, event.Value)
 	case feature.KeyUserSchema:
 		features.UserSchema.set(level, event.Value)
 	case feature.KeyTokenExchange: