all: require go1.21

Otherwise, code won't compile due to the use of slices package.

Fixes golang/go#68034

Change-Id: Id6fb27d2f213e5a665a2bcd6d07b15f80702975b
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/593235
Reviewed-by: Mauri de Souza Meneguzzo <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Cottrell <[email protected]>

go.mod

@@ -1,6 +1,6 @@
 module golang.org/x/vuln
 
-go 1.18
+go 1.21
 
 require (
 	github.com/google/go-cmdtest v0.4.1-0.20220921163831-55ab3332a786