chore: remove mysql57 #710

Job	Run time
run_ci (ubuntu-20.04, 3.11, discovery+lms+forum)	21m 33s
run_ci (ubuntu-20.04, 3.11, registrar+lms)	17m 3s
run_ci (ubuntu-20.04, 3.11, ecommerce+lms)	21m 23s
run_ci (ubuntu-20.04, 3.11, edx_notes_api+lms)	16m 17s
run_ci (ubuntu-20.04, 3.11, credentials+lms)	17m 44s
run_ci (ubuntu-20.04, 3.11, xqueue)	2m 39s
run_ci (ubuntu-20.04, 3.11, analyticsapi+insights+lms)	21m 37s
run_ci (ubuntu-20.04, 3.11, designer+lms)	17m 56s
run_ci (ubuntu-20.04, 3.11, enterprise-subsidy+enterprise-catalog+enterprise-access+lms)	23m 22s
run_ci (ubuntu-20.04, 3.11, license-manager+lms)	16m 56s
	2h 56m 30s

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