remove prohibition of unmanaged constructed types

[RexJaeschke]

[BillWagner]

rebased on the latest draft v8 branch on 9/26/2023

[jskeet]

The title talks about unmanaged constructed types - the text seems to be about pointer types. (It looks like constructed types still count as managed.) I think I need a bit more context to understand this change.

[jskeet]

(@jskeet to have another pass at this - in this PR.)

[jskeet]

And look for other places, e.g. 515.

[jskeet]

Also (try to) remove the grammar as type parameters can be unmanaged - instead, make references to it specify that the type must be unmanaged. (Do this in a separate commit.)