[CSM] Increase CPP PSM CSM timeout to 180min (#37154)

Pyhton timeout was already set to 240min:
3e8cdb9f39/tools/internal_ci/linux/psm-csm-python.cfg (L19)
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Closes #37154

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37154 from XuanWang-Amos:increase_csm_timeout f5caabc06a
PiperOrigin-RevId: 649187094

tools/internal_ci/linux/psm-csm.cfg

@@ -16,7 +16,7 @@
 
 # Location of the continuous shell script in repository.
 build_file: "grpc/tools/internal_ci/linux/psm-interop-test-cpp.sh"
-timeout_mins: 120
+timeout_mins: 180
 action {
   define_artifacts {
     regex: "artifacts/**/*sponge_log.xml"