Bump MSRV to 1.66

toml_edit requires it.

```
error: package `toml_edit v0.19.15` cannot be built because it requires rustc 1.66.0 or newer, while the currently active rustc version is 1.65.0
```

Author: taiki-e

Cargo.toml

@@ -2,7 +2,7 @@
 name = "cargo-hack"
 version = "0.6.5"
 edition = "2021"
-rust-version = "1.65"
+rust-version = "1.66"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/taiki-e/cargo-hack"
 keywords = ["cargo", "subcommand", "testing"]

README.md

@@ -431,7 +431,7 @@ fields of [`cargo metadata`][cargo-metadata].*
 cargo +stable install cargo-hack --locked
 ```
 
-*Compiler support: requires rustc 1.65+*
+*Compiler support: requires rustc 1.66+*
 
 cargo-hack is usually runnable with Cargo versions older than the Rust version
 required for installation (e.g., `cargo +1.31 hack check`). Currently, to run