Bump MSRV to 1.68 to deal with syn.

Author: emilio

.github/workflows/main.yml

@@ -18,7 +18,7 @@ jobs:
           - nightly
           - beta
           - stable
-          - 1.66.0
+          - 1.68.0
         features:
           -
           - --features dummy_match_byte

Cargo.toml

@@ -10,7 +10,7 @@ readme = "README.md"
 keywords = ["css", "syntax", "parser"]
 license = "MPL-2.0"
 edition = "2018"
-rust-version = "1.66"
+rust-version = "1.68"
 
 exclude = ["src/css-parsing-tests/**", "src/big-data-url.css"]