gopls/go.mod: update the go directive to 1.23.1

With Go 1.23.1 released, fixing some bugs in go/types Alias support, we
can now update the gopls go.mod go directive.

Fixes golang/go#65917

Change-Id: I2d6d3966de424048dceab1b2f7b795ec6998c863
Reviewed-on: https://go-review.googlesource.com/c/tools/+/610936
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/go.mod

@@ -1,6 +1,8 @@
 module golang.org/x/tools/gopls
 
-go 1.22.6 // => default GODEBUG has gotypesalias=0
+// go 1.23.1 fixes some bugs in go/types Alias support.
+// (golang/go#68894 and golang/go#68905).
+go 1.23.1
 
 require (
 	github.com/google/go-cmp v0.6.0