Expanding custom admin role support to "enforce MFA for admin" behavior

• Expand custom admin role support to https://docs.gitlab.com/security/two_factor_authentication/#enforce-2fa-for-administrator-users.
• Update the yellow banner from Nudge admin mode when creating a custom admin role (#555414 - closed) to recommend using the above feature as well. Messaging would only display for the features that weren’t enabled. If both features are enabled the banner is not displayed.