MediaLibraryService: Migration: Update didMigrate value

By caching it on the start on the medialibrary, we need to update
the value when successfully migrated, else it will continue to migrate
on each new media parsing.

SharedSources/MediaLibraryService.swift

@@ -233,6 +233,7 @@ private extension MediaLibraryService {
 
         if migrateMedia(oldMedialibrary) && migratePlaylists(oldMedialibrary) {
             UserDefaults.standard.set(true, forKey: MediaLibraryService.migrationKey)
+            didMigrate = true
             return true
         }
         return false