Bump crate version to 0.23.0

We just did an MSRV bump, this typically would require bumping our major
version number but since we are pre 1.0 we bump the minor version
number.

Bump crate version to the next minor version 0.23.0

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "secp256k1"
-version = "0.22.1"
+version = "0.23.0"
 authors = [ "Dawid Ciężarkiewicz <dpc@ucore.info>",
             "Andrew Poelstra <apoelstra@wpsoftware.net>" ]
 license = "CC0-1.0"