capi: update to Rust 2021

... and set an MSRV.

I usually don't bump MSRVs for small reasons, but this is moving to what
is still an extremely old version of Rust. And now we can inline format
arguments.

Author: BurntSushi

regex-capi/Cargo.toml

@@ -11,7 +11,8 @@ description = """
 A C API for Rust's regular expression library.
 """
 workspace = ".."
-edition = "2018"
+edition = "2021"
+rust-version = "1.65"
 
 [lib]
 name = "rure"