Bump MSRV to 1.76

```
  error: package `toml v0.9.7` cannot be built because it requires rustc 1.76 or newer, while the currently active rustc version is 1.75.0
  Either upgrade to rustc 1.76 or newer, or use
  cargo update toml@0.9.7 --precise ver
  where `ver` is the latest version of `toml` supporting rustc 1.75.0
```

Author: taiki-e

Cargo.toml

@@ -3,7 +3,7 @@ name = "cargo-hack"
 version = "0.6.38" #publish:version
 edition = "2021"
 # NB: Sync with the MSRV in "From source" section in README.md.
-rust-version = "1.75" # For ctrlc
+rust-version = "1.76" # For toml
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/taiki-e/cargo-hack"
 keywords = ["cargo", "subcommand", "testing"]

README.md

@@ -469,7 +469,7 @@ fields of [`cargo metadata`][cargo-metadata].*
 cargo +stable install cargo-hack --locked
 ```
 
-Currently, installing cargo-hack requires rustc 1.75+.
+Currently, installing cargo-hack requires rustc 1.76+.
 
 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