Update OAuth scopes for GitHub import

What does this MR do and why?

• fixes scopes so that collaborator import works and unnecessary scopes aren't asked for
• repo always required
• read:org required for optional collaborator import
• see: https://docs.gitlab.com/ee/user/project/import/github.html#select-additional-items-to-import
• Fixes #433091 (closed)

Screenshots or screen recordings

This is what the page looks like before a user has authenticated; clicking the blue button takes them through an OAuth flow where the scopes in our configuration are requested:

the oauth approval screen looks like this:

after authenticating, they are shown the import options:

How to set up and validate locally

• Configure GitHub OAuth locally https://docs.gitlab.com/ee/integration/github.html
• Visit GDK_ROOT/projects/new#import_project and select "GitHub"
• Click the blue button to authenticate with GitHub OAuth
• Confirm that scopes requested are repo and read:org

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.