update vet.sh to go runtime version 1.20

refs: google-cloud-go#7422
diff --git a/internal/kokoro/vet.sh b/internal/kokoro/vet.sh
index afdd25d..4e986cf 100755
--- a/internal/kokoro/vet.sh
+++ b/internal/kokoro/vet.sh
@@ -6,7 +6,7 @@
 # Display commands being run
 set -x
 
-if [[ $(go version) != *"go1.19"* ]]; then
+if [[ $(go version) != *"go1.20"* ]]; then
   exit 0
 fi