[lib] Update robotext for role changes

Summary:
Small change, the designs just indicate that a different robotext should be used for role changes.

Adding @ashoat as blocking since it involves copy changes

https://linear.app/comm/issue/ENG-4041/prevent-changing-the-only-admin-role-in-a-community

Depends on D8141

Test Plan:
Please see the updated robotext below

Reviewers: ashoat, atul, ginsu

Reviewed By: ashoat

Subscribers: tomek, ashoat

Differential Revision: https://phab.comm.dev/D8142