New feature flag form - Add optional field label

What does this MR do and why?

Update optional field description :

• Add (optional) to label
• Move it down

This ensures a better UX

Related issue

gitlab-org/developer-relations/contributor-success/contributors-gitlab-com#128 (closed)

Screenshots or screen recordings

Before	After

Steps to reproduce

Here's an improved English version of your instructions:

1. Start your GDK
2. Navigate to http://localhost:3000
3. Go to a project with feature flags enabled
4. Click "Deploy" to open a dropdown menu containing "Feature Flags" to click
5. Click "New feature flag"
6. Observe the form:
   • Optional fields should be labeled with "(optional)"
   • Required fields should not have "(optional)"
   • Inspect the HTML: required fields should have the attribute aria-required="true"