go.mod, compiler: bump minimum Go version to 1.23

This enables gotypesalias=1 by default, which is needed for
generic type aliases to work correctly in go/types.

testdata/typealias.go

@@ -0,0 +1,10 @@
+package main
+
+// Generic type alias (requires go 1.24+, or GOEXPERIMENT=aliastypeparams in go 1.23).
+type Set[T comparable] = map[T]struct{}
+
+func main() {
+	s := make(Set[string])
+	s["hello"] = struct{}{}
+	println(len(s))
+}

testdata/typealias.txt

@@ -0,0 +1 @@
+1