go.mod: change language version to 1.20

Allow use of 1.20 and later features in exp.

Change-Id: I85792cdc98d789028ed4fa222aac1c6b922e8b0d
Reviewed-on: https://go-review.googlesource.com/c/exp/+/486800
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/go.mod b/go.mod
index 0df6ab3..9443d49 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module golang.org/x/exp
 
-go 1.18
+go 1.20
 
 require (
 	github.com/google/go-cmp v0.5.8