docs: Remove DEFAULT from publishable keys env name in docs (#44696)

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

YES

## What kind of change does this PR introduce?

docs update

## What is the current behavior?

Inconsistent naming of publishable key environment variables across
dashboard, docs, ai skills, templates etc

## What is the new behavior?

Consistent naming of publishable key environment variables across
dashboard, docs, ai skills, templates etc


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

## Summary by CodeRabbit

* **Chores**
* Updated environment variable naming across example projects. Changed
`VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY` to
`VITE_SUPABASE_PUBLISHABLE_KEY` in configuration and initialization
files for the React quickstart and user-management examples.

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

Co-authored-by: Chris Chinchilla <chris.ward@supabase.io>

examples/auth/quickstarts/react/.env.example

@@ -1,2 +1,2 @@
 VITE_SUPABASE_URL=your-project-url
-VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY=your-publishable-key-or-anon-key
+VITE_SUPABASE_PUBLISHABLE_KEY=your-publishable-key-or-anon-key

examples/auth/quickstarts/react/src/App.jsx

@@ -4,7 +4,7 @@ import { createClient } from '@supabase/supabase-js'
 
 const supabase = createClient(
   import.meta.env.VITE_SUPABASE_URL,
-  import.meta.env.VITE_SUPABASE_PUBLISHABLE_DEFAULT_KEY
+  import.meta.env.VITE_SUPABASE_PUBLISHABLE_KEY
 )
 
 export default function App() {