gopls: update toolchain to go1.23.4

This fixes a serious miscompilation of range-over-func
(see golang/go#70035, and the symptom at CL 640035).

Change-Id: I843785a276c8181c5d6b799506d2808a5e7626ae
Reviewed-on: https://go-review.googlesource.com/c/tools/+/640036
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

gopls/go.mod

@@ -1,8 +1,8 @@
 module golang.org/x/tools/gopls
 
-// go 1.23.1 fixes some bugs in go/types Alias support.
-// (golang/go#68894 and golang/go#68905).
-go 1.23.1
+// go 1.23.1 fixes some bugs in go/types Alias support (golang/go#68894, golang/go#68905).
+// go 1.23.4 fixes a miscompilation of range-over-func (golang/go#70035).
+go 1.23.4
 
 require (
 	github.com/google/go-cmp v0.6.0