Bump MSRV to 1.56

```
error: failed to download `clap_lex v0.2.2`

Caused by:
  unable to get packages from source

Caused by:
  failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/clap_lex-0.2.2/Cargo.toml`

Caused by:
  feature `edition2021` is required

  this Cargo does not support nightly features, but if you
  switch to nightly channel you can add
  `cargo-features = ["edition2021"]` to enable this feature
```

.clippy.toml

@@ -1,4 +1,4 @@
-msrv = "1.54"
+msrv = "1.56"
 disallowed-methods = [
     # https://internals.rust-lang.org/t/synchronized-ffi-access-to-posix-environment-variable-functions/15475
     { path = "std::env::remove_var", reason = "this function should be considered `unsafe`" },

Cargo.toml

@@ -2,7 +2,7 @@
 name = "cargo-llvm-cov"
 version = "0.4.6" #publish:version
 edition = "2018"
-rust-version = "1.54"
+rust-version = "1.56"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/taiki-e/cargo-llvm-cov"
 keywords = ["cargo", "coverage", "subcommand", "testing"]

README.md

@@ -2,7 +2,7 @@
 
 [![crates.io](https://img.shields.io/crates/v/cargo-llvm-cov?style=flat-square&logo=rust)](https://crates.io/crates/cargo-llvm-cov)
 [![license](https://img.shields.io/badge/license-Apache--2.0_OR_MIT-blue?style=flat-square)](#license)
-[![rustc](https://img.shields.io/badge/rustc-1.54+-blue?style=flat-square&logo=rust)](https://www.rust-lang.org)
+[![rustc](https://img.shields.io/badge/rustc-1.56+-blue?style=flat-square&logo=rust)](https://www.rust-lang.org)
 [![build status](https://img.shields.io/github/workflow/status/taiki-e/cargo-llvm-cov/CI/main?style=flat-square&logo=github)](https://github.com/taiki-e/cargo-llvm-cov/actions)
 
 Cargo subcommand to easily use LLVM source-based code coverage.
@@ -457,7 +457,7 @@ Running cargo-llvm-cov requires rustc 1.60+.
 cargo install cargo-llvm-cov
 ```
 
-Currently, installing cargo-llvm-cov requires rustc 1.54+.
+Currently, installing cargo-llvm-cov requires rustc 1.56+.
 
 <!-- omit in toc -->
 ### From prebuilt binaries