bring the legacy config back

Signed-off-by: Juan Pablo Garcia Ripa <sarabadu@gmail.com>

docs/auth/service-to-service-auth.md

@@ -255,16 +255,10 @@ backend:
           subject: legacy-scaffolder
 ```
 
-:::warning
-The old style `backend.auth.keys` config is **no longer supported** and has been removed.
-If you are still using this configuration, you must migrate to the new `backend.auth.externalAccess` format above.
+The old style keys config is also supported as an alternative, but please
+consider using the new style above instead:
 
-You'll see an error like `"Unknown key 'backend.auth.keys'"` during Backstage startup if you haven't migrated yet.
-:::
-
-To migrate from the old config format:
-
-```yaml title="❌ Old format (no longer supported)"
+```yaml title="in e.g. app-config.production.yaml"
 backend:
   auth:
     keys:
@@ -272,22 +266,6 @@ backend:
       - secret: my-secret-key-scaffolder
 ```
 
-Convert to:
-
-```yaml title="✅ New format"
-backend:
-  auth:
-    externalAccess:
-      - type: legacy
-        options:
-          secret: my-secret-key-catalog
-          subject: external:backstage-plugin
-      - type: legacy
-        options:
-          secret: my-secret-key-scaffolder
-          subject: external:backstage-plugin
-```
-
 The secrets must be any base64-encoded random data, but for security reasons
 should be sufficiently long so as not to be easy to guess by brute force. You
 can for example generate them on the command line:
@@ -436,7 +414,7 @@ Each entry has one or more of the following fields:
 
 ## Adding custom or logic for validation and issuing of tokens
 
-The `pluginTokenHandlerDecoratorServiceRef` and `externalTokenTypeHandlersRef` can be used to extend the existing token handler without having to re-implement the entire `AuthService` implementation.
+The `pluginTokenHandlerDecoratorServiceRef` and `externalTokenHandlersServiceRef` can be used to extend the existing token handler without having to re-implement the entire `AuthService` implementation.
 This is particularly useful when you want to add additional logic to the handler, such as logging or metrics or custom token validation.
 
 ### PluginTokenHandler decoration
@@ -468,7 +446,7 @@ const decoratedPluginTokenHandler = createServiceFactory({
 
 ### Adding custom ExternalTokenHandler
 
-The `externalTokenTypeHandlersRef` can be used to add custom external token handlers to the default implementation.
+The `externalTokenHandlersServiceRef` can be used to add custom external token handlers to the default implementation.
 
 Your service factory must return an object with a `type` property that matches the token type in your configuration (e.g., 'custom', 'api-key'). When Backstage encounters tokens of this type, it calls your `initialize` method with all the configuration entries that match this type. Your factory can return either a single token handler or an array of handlers to process and validate these tokens.
 
@@ -503,13 +481,13 @@ And we can implement the custom token handler like this:
 ```ts
 import {
   ExternalTokenHandler,
-  externalTokenTypeHandlersRef,
+  externalTokenHandlersServiceRef,
   createExternalTokenHandler,
 } from '@backstage/backend-defaults/auth';
 import { createServiceFactory } from '@backstage/backend-plugin-api';
 
 const customExternalTokenHandlers = createServiceFactory({
-  service: externalTokenTypeHandlersRef,
+  service: externalTokenHandlersServiceRef,
   deps: {},
   async factory() {
     return createExternalTokenHandler({