Raise exception when nesting argument groups and mutually exclusive groups

[savannahostrowski]

The ability to nest argument groups and mutually exclusive groups was deprecated in 3.11 via #30098.

At present, we raise warnings to let users know this has been deprecated. I'd like to remove the logic since we're now two (going on three) releases from initial deprecation, and doing so will also simplify the logic to support new features, like #55797.

Linked PRs

• GH-127133: Remove ability to nest argument groups & mutually exclusive groups #127186