Fix flaky of `test_multi_echo` -- change sshd config to support large number of jobs #22225

Job	Run time
limited-deps-test (3.8, tests/test_jobs_and_serve.py)	26s
limited-deps-test (3.8, tests/test_cli.py)	1m 37s
failover-test (3.10, tests/test_failover.py)	1m 29s
python-test (3.8, tests/unit_tests)	2m 23s
python-test (3.8, tests/test_api.py)	1m 1s
python-test (3.8, tests/test_global_user_state.py)	52s
python-test (3.8, tests/test_list_accelerators.py)	1m 23s
python-test (3.8, tests/test_wheels.py)	1m 2s
python-test (3.8, tests/test_jobs.py)	3m 36s
python-test (3.8, tests/test_config.py)	1m 2s
python-test (3.8, tests/test_optimizer_random_dag.py)	1m 44s
python-test (3.8, tests/test_optimizer_dryruns.py)	8m 23s
python-test (3.8, tests/test_cli.py)	2m 7s
python-test (3.8, tests/test_jobs_and_serve.py)	1m 0s
python-test (3.8, tests/test_yaml_parser.py)	49s
python-test (3.8, tests/test_storage.py)	49s
	29m 43s

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