refactor: migrate STUDIO_REQUEST_EMAIL off FEATURES-as-dict

The v3 studio-home view still read the flag via
`settings.FEATURES.get('STUDIO_REQUEST_EMAIL', '')`. The earlier
STUDIO_REQUEST_EMAIL migration updated the v1 view
(contentstore/rest_api/v1/views/home.py) and course_creators/admin.py to
flat `settings.STUDIO_REQUEST_EMAIL`, but missed this duplicated v3 copy.
The FeaturesProxy bridge kept it working, so CI stayed green.

Read the flag off flat settings with `settings.STUDIO_REQUEST_EMAIL`
(defined in cms/envs/common.py, default `''` — matching the prior `.get`
default). Tests already override it flat via
`@override_settings(..., STUDIO_REQUEST_EMAIL=...)`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: feanil

cms/djangoapps/contentstore/rest_api/v3/views/home.py

@@ -122,7 +122,7 @@ def list(self, request: Request):
             ),
             'studio_name': settings.STUDIO_NAME,
             'studio_short_name': settings.STUDIO_SHORT_NAME,
-            'studio_request_email': settings.FEATURES.get('STUDIO_REQUEST_EMAIL', ''),
+            'studio_request_email': settings.STUDIO_REQUEST_EMAIL,
             'tech_support_email': settings.TECH_SUPPORT_EMAIL,
             'platform_name': settings.PLATFORM_NAME,
             'user_is_active': request.user.is_active,