Capitalize Client in "client modules"

[Huxpro]

Summary of changes:
See #6417.

Alternatively, we can follow the current Next.js doc and use "Client Component modules". However, considering the possible existence of React Client Action, it seems like this notion does not necessarily bound to "Component".

How do we call this:

// requestUsername.js
'use server';

export default async function requestUsername(formData) {
  const username = formData.get('username');
  if (canRequest(username)) {
    // ...
    return 'successful';
  }
  return 'failed';
}

• Server module?
• Server Action module?
• Server Module??

[github-actions]

Size changes

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌