Upgrade to 2021 edition of Rust

To do so, we need to bump the MSRV to 1.56 (released 10/2021). While at
it, I actually bumped it all the way to 1.62, where the latest version,
(1.62.1) has been released on July 19, which is just more than one month
ago. Additionally, I removed `clippy.toml` and instead specified the
MSRV in `Cargo.toml`. This will also print an error when trying to
compile with an older compiler.

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         platform: [ubuntu-latest, macos-latest, windows-latest]
-        rust: ['1.54', stable]
+        rust: ['1.62', stable]
     runs-on: ${{ matrix.platform }}
     steps:
       - uses: actions/checkout@v2

Cargo.toml

@@ -12,7 +12,8 @@ repository = "https://github.com/dbrgn/tealdeer/"
 documentation = "https://dbrgn.github.io/tealdeer/"
 version = "1.5.0"
 include = ["/src/**/*", "/tests/**/*", "/Cargo.toml", "/README.md", "/LICENSE-*", "/screenshot.png"]
-edition = "2018"
+rust-version = "1.62"
+edition = "2021"
 
 [[bin]]
 name = "tldr"

clippy.toml

@@ -1 +0,0 @@
-msrv = "1.54"