syntax: switch to Rust 2021

This effectively bumps the MSRV of 'regex' to Rust 1.56, which was
released in Oct 2021. It's not quite a year at the time of writing, but
I expect it will be a year by the time this change is released.

regex-syntax/Cargo.toml

@@ -8,7 +8,7 @@ documentation = "https://docs.rs/regex-syntax"
 homepage = "https://github.com/rust-lang/regex"
 description = "A regular expression parser."
 workspace = ".."
-edition = "2018"
+edition = "2021"
 
 # Features are documented in the "Crate features" section of the crate docs:
 # https://docs.rs/regex-syntax/*/#crate-features