Update psm-dualstack.cfg (#11950) (#12031)

120 minutes has not been sufficient, causing frequent VM timeout errors in the test runs.

Author: kannanjgithub

buildscripts/kokoro/psm-dualstack.cfg

@@ -2,7 +2,7 @@
 
 # Location of the continuous shell script in repository.
 build_file: "grpc-java/buildscripts/kokoro/psm-interop-test-java.sh"
-timeout_mins: 120
+timeout_mins: 240
 
 action {
   define_artifacts {