Support "Minimal access" role when inviting new users

What does this MR do and why?

Adds Minimal Access role to list of available options when inviting a user into a group (without parent group).

Fixes Cannot invite new members with 'Minimal Access'... (#353816 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

0. Make sure feature minimal_access_role is available (requires Premium license)

# bin/rails console
pry(main)> License.feature_available?(:minimal_access_role)
=> true

1. Visit a groups "Group members" page (for example, http://gdk.test:3000/groups/flightjs/-/group_members)
2. Click "Invite members"
3. Click the "Select a role" dropdown
4. Check to see "Minimal Access" is now available as an option

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cc @prajnamas @zhzhang93