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 dd5c45c..15b9f2e 100755
--- a/internal/kokoro/vet.sh
+++ b/internal/kokoro/vet.sh
@@ -7,7 +7,7 @@
 # Fail on error, and display commands being run.
 set -ex
 
-if [[ $(go version) != *"go1.19"* ]]; then
+if [[ $(go version) != *"go1.20"* ]]; then
   exit 0
 fi