Bump rack from 3.1.18 to 3.1.20 in /ruby (#7750) #133

Job	Run time
run_job_with_aws (ruby)	17s
run_job_with_aws (kotlin)	23s
run_job_with_aws (steering_docs)	30s
run_job_with_aws (rustv1)	18s
run_job_with_aws (javascriptv3)	23s
run_job_with_aws (javav2)	30s
run_job_with_aws (coding-standards)	32s
run_job_with_aws (cpp)	23s
run_job_with_aws (gov2)	19s
run_job_with_aws (swift)	28s
run_job_with_aws (dotnetv4)	53s
run_job_with_aws (specs)	24s
run_job_with_aws (python)	20s
run_job_with_aws (php)	15s
	5m 55s

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