docs: 3rd party and SSO Keys changes (#45167)

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Updated Flutter authentication guides (Auth0, AWS Cognito, Clerk, and
Firebase Auth) with corrected Supabase client initialization parameters
in code examples.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/docs/content/guides/auth/third-party/auth0.mdx

@@ -88,7 +88,7 @@ Future<void> main() async {
   final auth0 = Auth0('AUTH0_DOMAIN', 'AUTH0_CLIENT_ID');
   await Supabase.initialize(
     url: 'https://<supabase-project>.supabase.co',
-    anonKey: 'SUPABASE_PUBLISHABLE_KEY',
+    publishableKey: 'SUPABASE_PUBLISHABLE_KEY',
     accessToken: () async {
       final credentials = await auth0.credentialsManager.credentials();
       return credentials.idToken;

apps/docs/content/guides/auth/third-party/aws-cognito.mdx

@@ -89,7 +89,7 @@ import 'package:supabase_flutter/supabase_flutter.dart';
 Future<void> main() async {
   await Supabase.initialize(
     url: 'https://<supabase-project>.supabase.co',
-    anonKey: 'SUPABASE_PUBLISHABLE_KEY',
+    publishableKey: 'SUPABASE_PUBLISHABLE_KEY',
     accessToken: () async {
       final session = await Amplify.Auth.fetchAuthSession();
       final cognitoSession = session as CognitoAuthSession;

apps/docs/content/guides/auth/third-party/clerk.mdx

@@ -56,7 +56,7 @@ import 'package:supabase_flutter/supabase_flutter.dart';
 
 await Supabase.initialize(
   url: 'SUPABASE_URL',
-  anonKey: 'SUPABASE_PUBLISHABLE_KEY',
+  publishableKey: 'SUPABASE_PUBLISHABLE_KEY',
   accessToken: () async {
     final token = await ClerkAuth.of(context).sessionToken();
     return token.jwt;

apps/docs/content/guides/auth/third-party/firebase-auth.mdx

@@ -49,7 +49,7 @@ Creating a client for the Web is as easy as passing the `accessToken` async func
 ```dart
 await Supabase.initialize(
   url: supabaseUrl,
-  anonKey: supabaseKey,
+  publishableKey: publishableKey,
   debug: false,
   accessToken: () async {
     final token = await FirebaseAuth.instance.currentUser?.getIdToken();