Remove toolchain directive from library

Discussing this with @williammartin, neither of us could reason why the `toolchain` directive was necessary for a library and choosing to remove.

Author: andyfeller

go.mod

@@ -2,8 +2,6 @@ module github.com/cli/go-gh/v2
 
 go 1.23.0
 
-toolchain go1.23.7
-
 require (
 	github.com/AlecAivazis/survey/v2 v2.3.7
 	github.com/MakeNowJust/heredoc v1.0.0