Explicitly set "rust-version" in Cargo.toml. Upgrade Rust version in clippy.toml: 1.58.1 -> 1.86.

Documentation:
- https://github.com/rust-lang/rust-clippy?tab=readme-ov-file#specifying-the-minimum-supported-rust-version
- https://doc.rust-lang.org/clippy/configuration.html#specifying-the-minimum-supported-rust-version
- https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field
- https://doc.rust-lang.org/cargo/reference/rust-version.html

Setting "rust-version" in Cargo.toml might be redundant for Clippy, but
it may serve well for Cargo itself. So, I set it in both places.

Changes:
- Update "msrv" in clippy.toml: 1.58.1 -> 1.86.
- Explicitly set "rust-version" in Cargo.toml to "1.86".

clippy.toml

@@ -1,5 +1,5 @@
 # Disabled until we use at least Rust 1.49.0, which is the first
 # version that supports the msrv field.
-msrv = "1.58.1"
+msrv = "1.86"
 
 too-many-arguments-threshold = 10