Automatic toolchain upgrade to nightly-2024-07-21 (rust-lang#3367)

Update Rust toolchain from nightly-2024-07-20 to nightly-2024-07-21
without any other source changes.
This is an automatically generated pull request. If any of the CI checks
fail, manual intervention is required. In such a case, review the
changes at https://github.com/rust-lang/rust from
rust-lang@9057c3f
up to
rust-lang@5069856.

Author: github-actions[bot]

rust-toolchain.toml

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: Apache-2.0 OR MIT
 
 [toolchain]
-channel = "nightly-2024-07-20"
+channel = "nightly-2024-07-21"
 components = ["llvm-tools", "rustc-dev", "rust-src", "rustfmt"]