Update MSRV 1.46 -> 1.56

Author: nph4rd

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
           submodules: recursive
     - uses: actions-rs/toolchain@v1
       with:
-          toolchain: 1.46.0
+          toolchain: 1.56.0
           override: true
     - uses: actions-rs/cargo@v1
       with:

Cargo.toml

@@ -2,7 +2,7 @@
 name = "plotters"
 version = "0.3.1"
 authors = ["Hao Hou <[email protected]>"]
-edition = "2018"
+edition = "2021"
 license = "MIT"
 description = "A Rust drawing library focus on data plotting for both WASM and native applications"
 repository = "https://github.com/38/plotters"

README.md

@@ -19,7 +19,7 @@
     <img style="display: inline! important" src="https://codecov.io/gh/38/plotters/branch/master/graph/badge.svg" />
 </a>
 
-![Minimum Rust version 1.46](https://img.shields.io/badge/rustc-1.46+-06a096.svg)
+![Minimum Rust version 1.56](https://img.shields.io/badge/rustc-1.56+-06a096.svg)
 
 Plotters is drawing library designed for rendering figures, plots, and charts, in pure rust. Plotters supports various types of back-ends, 
 including bitmap, vector graph, piston window, GTK/Cairo and WebAssembly.