Add `available_from_access_level` property to custom role abilities

What does this MR do and why?

The available_from_access_level property defines the access level from which the ability is available, when applicable.

It is exposed in the GraphQL API on the CustomizablePermissionType.

Issue: #426740 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. create a new ability by running the following command

   ee/bin/custom-ability new_custom_ability -f -d 'ability description' -c 'ability category' -M '16.9' -m 'https://mr' -i 'https://issue' -g -p -r read_code

2. restart rails

   gdk restart rails-web

3. Visit http://localhost:3000/-/graphql-explorer and run the following query:

   query {
     memberRolePermissions {
       nodes {
         name
         availableFromAccessLevel {
           integerValue
           stringValue
         }
       }
     }
   }

4. Verify the new ability is returned with the correct access level from which it is available