chore(go): Add warning about main branch

Main branch is in a state of flux because of the partitioned cookie
attribute which won't land in go until version 1.23. Added a
warning to this affect as well as upped the version of go in the
go.mod to 1.23 to indicate this change.

README.md

@@ -1,4 +1,7 @@
-# sessions
+# Gorilla Sessions
+
+> [!CAUTION]
+> The main branch of this respository is in flux and being prepared for the [Go 1.23 release](https://github.com/golang/go/milestone/212). Please use released versions as per the release page.
 
 ![testing](https://github.com/gorilla/sessions/actions/workflows/test.yml/badge.svg)
 [![codecov](https://codecov.io/github/gorilla/sessions/branch/main/graph/badge.svg)](https://codecov.io/github/gorilla/sessions)

go.mod

@@ -1,5 +1,5 @@
 module github.com/gorilla/sessions
 
-go 1.20
+go 1.23
 
 require github.com/gorilla/securecookie v1.1.2