Display which role is the 'Administrator role' on /admin/people/roles

Problem/Motivation

Follow-up for #987978-57: Move "administrator role" setting to new Role Settings form → :

we probably want to visually promote the value to the role overview page. Sort of the "settings summary" pattern. Or a whole new column in that table (there's plenty of space for it) to indicate anything special about each role. Not sure what the column should be labeled, what values it would contain, etc. But if I'm scanning my collection of roles, it'd be nice to know which one(s) are the admin role(s)

Steps to reproduce

1. Install core (any version D8/9):
2. Visit /admin/people/roles
3. Wonder which one is the 'Administrator role'

Proposed resolution

Somehow indicate which role at /admin/people/roles is the 'Administrator role'.

Remaining tasks

1. Decide on the UI
2. Implement
3. Add / update tests
4. Reviews / refinements
5. RTBC
6. Commit

User interface changes

TBD.

API changes

Unlikely.

Data model changes

None.

Release notes snippet

TBD.