Update the rust toolchain to nightly-2024-04-21

This update will fix the below error in AnalysisHub.

```
$ ./scripts/deps/rust.sh
...
   Compiling tempfile v3.10.1
   Compiling serde_derive v1.0.201
error[E0658]: use of unstable library feature 'saturating_int_impl'
   --> /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.201/src/lib.rs:280:13
    |
280 |     pub use self::core::num::Saturating;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #87920 <rust-lang/rust#87920> for more information
    = help: add `#![feature(saturating_int_impl)]` to the crate attributes to enable
...
```

Signed-off-by: Changho Choi <[email protected]>

rust-toolchain

@@ -1 +1 @@
-nightly-2023-09-05
+nightly-2024-04-21