core-plugin-api: add forwards compatibility for route refs

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/frontend-system/building-plugins/05-migrating.md

@@ -38,7 +38,6 @@ In order to migrate the actual definition of the plugin you need to recreate the
 
 ```ts title="my-plugin/src/alpha.tsx"
   import { createFrontendPlugin } from '@backstage/frontend-plugin-api';
-  import { convertLegacyRouteRefs } from '@backstage/core-compat-api';
 
   export default createFrontendPlugin({
     // The plugin ID is now provided as `pluginId` instead of `id`
@@ -47,15 +46,12 @@ In order to migrate the actual definition of the plugin you need to recreate the
     // bind all the extensions to the plugin
     /* highlight-next-line */
     extensions: [/* APIs will go here, but don't worry about those yet */],
-    // convert old route refs to the new system
-    /* highlight-next-line */
-    routes: convertLegacyRouteRefs({
+    routes: {
       ...
-    }),
-    /* highlight-next-line */
-    externalRoutes: convertLegacyRouteRefs({
+    },
+    externalRoutes: {
       ...
-    }),
+    },
   });
 ```
 
@@ -110,18 +106,15 @@ it can be migrated as the following, keeping in mind that you may need to switch
 
 ```tsx
 import { PageBlueprint } from '@backstage/frontend-plugin-api';
-import {
-  compatWrapper,
-  convertLegacyRouteRef,
-} from '@backstage/core-compat-api';
+import { compatWrapper } from '@backstage/core-compat-api';
 
 const fooPage = PageBlueprint.make({
   params: {
     // This is the path that was previously defined in the app code.
     // It's labelled as the default one because it can be changed via configuration.
     path: '/foo',
-    // You can reuse the existing routeRef by wrapping it with convertLegacyRouteRef.
-    routeRef: convertLegacyRouteRef(rootRouteRef),
+    // You can reuse the existing routeRef.
+    routeRef: rootRouteRef,
     // these inputs usually match the props required by the component.
     loader: () =>
       import('./components/').then(m =>