Remove code that relies on v4 cookie redirects (#10501)

Co-authored-by: Sarah Rainsberger <[email protected]>

src/content/docs/en/guides/actions.mdx

@@ -501,7 +501,7 @@ const result = Astro.getActionResult(actions.newsletter);
 const inputErrors = isInputError(result?.error) ? result.error.fields : {};
 ---
 
-<form method="POST" action={'/confirmation' + actions.newsletter}>
+<form method="POST" action={actions.newsletter}>
   <label>
     E-mail
     <input required type="email" name="email" aria-describedby="error" />