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/api/grpc/feature/v2beta/integration_test/feature_test.go

@@ -194,10 +194,6 @@ func TestServer_GetInstanceFeatures(t *testing.T) {
 					Enabled: false,
 					Source:  feature.Source_SOURCE_UNSPECIFIED,
 				},
-				OidcLegacyIntrospection: &feature.FeatureFlag{
-					Enabled: false,
-					Source:  feature.Source_SOURCE_UNSPECIFIED,
-				},
 				UserSchema: &feature.FeatureFlag{
 					Enabled: false,
 					Source:  feature.Source_SOURCE_UNSPECIFIED,
@@ -256,10 +252,6 @@ func TestServer_GetInstanceFeatures(t *testing.T) {
 					Enabled: false,
 					Source:  feature.Source_SOURCE_UNSPECIFIED,
 				},
-				OidcLegacyIntrospection: &feature.FeatureFlag{
-					Enabled: false,
-					Source:  feature.Source_SOURCE_UNSPECIFIED,
-				},
 				UserSchema: &feature.FeatureFlag{
 					Enabled: false,
 					Source:  feature.Source_SOURCE_UNSPECIFIED,
@@ -285,7 +277,6 @@ func TestServer_GetInstanceFeatures(t *testing.T) {
 			require.NoError(t, err)
 			assertFeatureFlag(t, tt.want.LoginDefaultOrg, got.LoginDefaultOrg)
 			assertFeatureFlag(t, tt.want.OidcTriggerIntrospectionProjections, got.OidcTriggerIntrospectionProjections)
-			assertFeatureFlag(t, tt.want.OidcLegacyIntrospection, got.OidcLegacyIntrospection)
 			assertFeatureFlag(t, tt.want.UserSchema, got.UserSchema)
 		})
 	}