[Turbopack] change persistent caching option to a boolean (#72458)

### What?

Change the option to a boolean, the number version is weird and unexpected.

Closes PACK-3418

packages/next/src/server/config-shared.ts

@@ -152,10 +152,8 @@ export interface ExperimentalTurboOptions {
 
   /**
    * Enable persistent caching for the turbopack dev server and build.
-   * Need to provide the expected level of stability, otherwise it will fail.
-   * Currently stability level: 1
    */
-  unstablePersistentCaching?: number | false
+  unstablePersistentCaching?: boolean
 
   /**
    * Enable tree shaking for the turbopack dev server and build.

packages/next/src/server/dev/turbopack-utils.ts

@@ -1205,11 +1205,5 @@ export function normalizedPageToTurbopackStructureRoute(
 export function isPersistentCachingEnabled(
   config: NextConfigComplete
 ): boolean {
-  const unstableValue = config.experimental.turbo?.unstablePersistentCaching
-  if (typeof unstableValue === 'number' && unstableValue > 1) {
-    throw new Error(
-      'Persistent caching in this version of Turbopack is not as stable as expected. Upgrade to a newer version of Turbopack to use this feature with the expected stability.'
-    )
-  }
-  return !!unstableValue
+  return config.experimental.turbo?.unstablePersistentCaching || false
 }