Update the rust toolchain to nightly-2024-04-02 (model-checking#3127)

Changes required due to:
- rust-lang/rust@a325bce3cd Normalize the result of Fields::ty_with_args

Resolves: model-checking#3125, model-checking#3113

tests/kani/ValidValues/write_invalid_fixme.rs → tests/kani/ValidValues/write_invalid.rs

@@ -6,9 +6,6 @@
 //! Writing invalid bytes is not UB as long as the incorrect value is not read.
 //! However, we over-approximate for sake of simplicity and performance.
 
-// Note: We're getting an unexpected compilation error because the type returned
-// from StableMIR is `Alias`: https://github.com/model-checking/kani/issues/3113
-
 use std::num::NonZeroU8;
 
 #[kani::proof]