go.mod: change version to 1.20

bigwhite · bigwhite · commit 9cd8c5d88745 · 2023-02-05T13:43:29.000+08:00
diff --git a/go.mod b/go.mod
@@ -1,6 +1,6 @@
 module github.com/bigwhite/experiments
 
-go 1.15
+go 1.20
 
 require (
 	bitbucket.org/bigwhite/m2 v1.0.2 // indirect
