Raise the MSRV to 1.56.1

This is needed as nix v0.26.0 raised its MSRV to 1.56.1. For more
information please see the changelog:
	https://github.com/nix-rust/nix/blob/master/CHANGELOG.md#0260---2022-11-29

Or the corresponding PR:
	nix-rust/nix#1792

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>

.github/workflows/ci.yaml

@@ -56,7 +56,7 @@ jobs:
       disable_tests: true
       extra_packages: libudev-dev
       target: x86_64-unknown-linux-gnu
-      toolchain: "1.46.0"
+      toolchain: "1.56.1"
 
   # --------------------------------------------------------------------------
   # BUILD

README.md

@@ -100,7 +100,7 @@ can help debug software or hardware errors.
 
 # Dependencies
 
-Rust versions 1.46.0 and higher are supported.
+Rust versions 1.56.1 and higher are supported.
 
 For GNU Linux `pkg-config` headers are required: