Bump to 2021 edition

.github/workflows/msrv.yml

@@ -10,10 +10,10 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: 1.46.0 # bitflags requires 1.46+
+          toolchain: 1.56.0 # 2021 edition requires 1.56
           override: true
       - uses: actions/checkout@v2
-      - name: cargo +1.46.0 check
+      - name: cargo +1.56.0 check
         uses: actions-rs/cargo@v1
         with:
           command: check

Cargo.toml

@@ -8,7 +8,7 @@ repository = "https://github.com/jonhoo/rust-imap"
 homepage = "https://github.com/jonhoo/rust-imap"
 description = "IMAP client for Rust"
 license = "Apache-2.0 OR MIT"
-edition = "2018"
+edition = "2021"
 
 keywords = ["email", "imap"]
 categories = ["email", "network-programming"]