Flutter Firestore offline setup uses depreciate code

[srivats22]

In the Flutter Firestore Plugin Using the following code:

await db
        .enablePersistence(const PersistenceSettings(synchronizeTabs: true));

Throws a warning saying enablePersistence is depreciated and should use persistenceEnabled.

snippets-flutter/packages/firebase_snippets_app/lib/snippets/firestore.dart

Lines 1072 to 1077 in 0428b74

	// Apple and Android
	db.settings = const Settings(persistenceEnabled: true);
	// Web
	await db
	.enablePersistence(const PersistenceSettings(synchronizeTabs: true));

Can the document be updated to reflect this? I can submit a PR for the same if that's ok.