Fix: Remove shared_runners_setting attribute from groups creation API

What does this MR do and why?

Fix: Allow shared_runners_setting attribute only for groups update API

Changelog: fixed

References

• Part of A 500 error was encountered when executing the ... (#586605 - closed).

How to set up and validate locally

• Run GDK.
• Confirm it succeeds to create a new group with shared_runners_setting attribute. This should succeed without error message, because Rails ignores invalid params and this is consistent with other API endpoints.

$ curl -k -X POST "https://gdk.test:3443/api/v4/groups?path=issue-586605-group-4" \
  -H "Authorization: Bearer $PAT" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "586605-group-2",
    "visibility": "private",
    "share_with_group_lock": true,
    "shared_runners_setting": "invalid_value"
  }'

Response includes:

{
  "id":201,
  "web_url":"https://gdk.test:3443/groups/issue-586605-group-4",
  ...
  "shared_runners_setting":"enabled",
  ...
}

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