Mark unstable_cache as legacy for better understanding

Clarify that `unstable_cache` is a legacy API and recommend using Cache Components instead.

Author: kostyniuk

docs/01-app/01-getting-started/09-caching-and-revalidating.mdx

@@ -224,7 +224,7 @@ export async function updateUser(id) {
 
 See the [`revalidatePath` API reference](/docs/app/api-reference/functions/revalidatePath) to learn more.
 
-## `unstable_cache`
+## `unstable_cache` (Legacy)
 
 > **Good to know**: `unstable_cache` is an experimental API. We recommend opting into [Cache Components](/docs/app/getting-started/cache-components) and replacing `unstable_cache` with the [`use cache`](/docs/app/api-reference/directives/use-cache) directive. See the [Cache Components documentation](/docs/app/getting-started/cache-components) for more details.