docs: drop the retired withSupabase middleware option from the intro (#50409)

## 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 fix, one sentence removed from the `@supabase/middleware` reference
intro.

## What is the current behavior?

The alpha admonition says the `middleware` option on `withSupabase` in
`@supabase/server` is also alpha. That option was removed in
`@supabase/server` 1.6.0, where `withSupabase` became a `pipeline`
entry, so the sentence describes something that no longer exists.

## What is the new behavior?

The admonition keeps the `@supabase/middleware` alpha wording and drops
the sentence about the removed option. Both hand-maintained copies of
the intro are updated and stay identical: the spec partial that renders
the reference site, and the `docs/ref` copy that feeds the markdown
build.

## Additional context

Same two-file pattern the `@supabase/server` reference uses for its
intro and installing partials.

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

## Summary by CodeRabbit

- **Documentation**
- Updated middleware documentation to clarify that only the
`@supabase/middleware` package is in alpha.
- Removed the alpha-status notice for the `withSupabase` middleware
option in `@supabase/server`.
- Clarified that `@supabase/middleware` APIs may change between 0.x
releases.

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

apps/docs/spec/reference/middleware/v1/partials/introduction.mdx

@@ -9,7 +9,7 @@ Each middleware can guard the request, edit the response, or contribute typed va
 
 <Admonition type="caution" title="Alpha">
 
-`@supabase/middleware` is in alpha. APIs may change between 0.x releases. The `middleware` option on `withSupabase` in `@supabase/server` is also alpha.
+`@supabase/middleware` is in alpha. APIs may change between 0.x releases.
 
 </Admonition>