Rename PSM interop fallback test suite to light (#12092)

Author: murgatroid99

buildscripts/kokoro/psm-fallback.cfg renamed to buildscripts/kokoro/psm-light.cfg

@@ -13,5 +13,5 @@ action {
 }
 env_vars {
   key: "PSM_TEST_SUITE"
-  value: "fallback"
+  value: "light"
 }