readme: require go 1.21

Author: markus-wa

README.md

@@ -55,7 +55,7 @@ For business inquiries please use the contact information found on the [GitHub p
 
 ## Requirements
 
-This library requires at least `go 1.18` to run.
+This library requires at least `go 1.21` to run.
 You can download the latest version of Go [here](https://golang.org/).
 
 ## Quickstart Guide