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/projection/instance_features_test.go

@@ -26,7 +26,7 @@ func TestInstanceFeaturesProjection_reduces(t *testing.T) {
 			args: args{
 				event: getEvent(
 					testEvent(
-						feature_v2.InstanceLegacyIntrospectionEventType,
+						feature_v2.SystemUserSchemaEventType,
 						feature_v2.AggregateType,
 						[]byte(`{"value": true}`),
 					), eventstore.GenericEventMapper[feature_v2.SetEvent[bool]]),
@@ -41,7 +41,7 @@ func TestInstanceFeaturesProjection_reduces(t *testing.T) {
 							expectedStmt: "INSERT INTO projections.instance_features2 (instance_id, key, creation_date, change_date, sequence, value) VALUES ($1, $2, $3, $4, $5, $6) ON CONFLICT (instance_id, key) DO UPDATE SET (creation_date, change_date, sequence, value) = (projections.instance_features2.creation_date, EXCLUDED.change_date, EXCLUDED.sequence, EXCLUDED.value)",
 							expectedArgs: []interface{}{
 								"agg-id",
-								"legacy_introspection",
+								"user_schema",
 								anyArg{},
 								anyArg{},
 								uint64(15),