Customizable roles cannot be edited when assigned

What does this MR do and why?

Solves #375661 (closed)

Screenshots or screen recordings

Relevant specs have been added to test the change.

Screenshot from rails console

How to set up and validate locally

1. Create a new member_role for a root level group

   curl --request POST --header "Content-Type: application/json" --header "Authorization: Bearer $YOUR_ACCESS_TOKEN" --data '{"base_access_level":10, "download_code":1}' "https://gdk.test:3443/api/v4/groups/$GROUP_PATH/member_roles"

2. In rails console assign any member newly created member_role

    group = Group.find_by(path: '$GROUP_PATH')
    user = User.find_by(email: '$TEST_USER_EMAIL')
    member_role = group.member_roles.first
    member = Member.where(user: user, source: group).first
    member.update!(member_role: member_role)

3. Try and update any attribute for new member_role and an exception should be raised

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.