Separate `view code` permission from `download code` - create 2 permissions

Background

In Allow Ultimate Guests to view repository conten... (#20277 - closed), the ~"group::authentication and authorization" has a request to allow Guest Users with an Ultimate license to view code.

Today, as stated here, read code and download code are lumped into one permission. This makes it impossible to implement the feature request, because we only want to allow read code.

Proposal

• Separate read code permission from download code - this will create 2 permissions from the current one
• Ensure that this separation is transparent to the end user and does not impact their current abilities to view or download code.
• Users who have the ability to download code automatically have the ability to view code