test/kokoro: increase PSM Security test timeout to 4h (#6193)

diff --git a/test/kokoro/psm-security.cfg b/test/kokoro/psm-security.cfg
index 7d86de6..040efe9 100644
--- a/test/kokoro/psm-security.cfg
+++ b/test/kokoro/psm-security.cfg
@@ -2,7 +2,7 @@
 
 # Location of the continuous shell script in repository.
 build_file: "grpc-go/test/kokoro/psm-security.sh"
-timeout_mins: 180
+timeout_mins: 240
 
 action {
   define_artifacts {