[Kokoro] Increase arm64 test timeout (#32950)

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tools/internal_ci/linux/arm64/grpc_bazel_test_c_cpp.cfg

@@ -16,7 +16,7 @@
 
 # Location of the continuous shell script in repository.
 build_file: "grpc/tools/internal_ci/linux/arm64/grpc_bazel.sh"
-timeout_mins: 150
+timeout_mins: 300
 action {
   define_artifacts {
     regex: "**/*sponge_log.*"