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>

cmd/start/config_test.go

@@ -85,7 +85,6 @@ Actions:
 		want: func(t *testing.T, config *Config) {
 			assert.Equal(t, config.DefaultInstance.Features, &command.InstanceFeatures{
 				LoginDefaultOrg:                 gu.Ptr(true),
-				LegacyIntrospection:             gu.Ptr(true),
 				TriggerIntrospectionProjections: gu.Ptr(true),
 				UserSchema:                      gu.Ptr(true),
 			})