Bump gcc from 12.2.0 to 15.2.0 in /cpp_grpc_mt_bench #985

Job	Run time
meta-check	4s
set-image-name	2s
changed	5s
cpp_grpc_mt_bench	35s
cpp_grpc_st_bench	0s
java_armeria_bench	0s
cpp_asio_grpc_bench	0s
dotnet_grpc_bench	0s
go_vtgrpc_bench	0s
crystal_grpc_bench	0s
java_micronaut_bench	0s
go_vtconnect_bench	0s
go_connect_bench	0s
haskell_grpc_haskell_bench	0s
java_micronaut_workstealing_bench	0s
java_aot_bench	0s
java_hotspot_grpc_she_bench	0s
d_grpc_bench	0s
java_hotspot_grpc_g1gc_bench	0s
java_hotspot_grpc_pgc_bench	0s
erlang_grpcbox_bench	0s
java_hotspot_grpc_sgc_bench	0s
java_hotspot_grpc_zgc_bench	0s
go_grpc_bench	0s
java_quarkus_bench	0s
java_openj9_grpc_gencon_bench	0s
scala_zio_bench	0s
node_grpcjs_st_bench	0s
swift_grpc_bench	0s
rust_tonic_st_bench	0s
php_grpc_bench	0s
python_grpc_bench	0s
kotlin_grpc_bench	0s
rust_tonic_mt_bench	0s
rust_thruster_st_bench	0s
python_async_grpc_bench	0s
rust_pajamax_bench	0s
java_vertx_grpc_bench	0s
scala_fs2_bench	0s
rust_thruster_mt_bench	0s
ruby_grpc_bench	0s
scala_pekko_bench	0s
java_quarkus_native_bench	0s
rust_grpcio_bench	0s
scala_akka_bench	0s
rust_wtx_bench	0s
	46s

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