This PR renames `UInt8.ofNatTruncate` to `UInt8.ofNatClamp`.
The new name is more consistent with the rest of the library (e.g.,
`Int8.toNatClampNeg`) and also more clear. See also [#lean4 > Name of
UInt32.ofNatTruncate etc may be
wrong](https://leanprover.zulipchat.com/#narrow/channel/270676-lean4/topic/Name.20of.20UInt32.2EofNatTruncate.20etc.20may.20be.20wrong/with/591873604).
Historical note: during prehistoric times (in January 2024),
`UInt32.ofNatTruncate` (only the 32 bit version) was introduced for use
in the `simp` configuration. When I looked at the `UIntX` API more
properly later, I added the missing variants but didn't reconsider the
name.
ba502f7027