update core to 5.2.1 (#1236) #1684

Job	Run time
e2e_tests (latest, 7.2)	16m 33s
unit_tests (latest, 7.2, singlesite)	12m 46s
e2e_tests (latest, 8.1)	16m 45s
checkstyle	28s
unit_tests (latest, 7.2, multisite)	13m 23s
e2e_tests (trunk, 7.2)	16m 47s
unit_tests (latest, 8.1, singlesite)	13m 46s
e2e_tests (trunk, 8.1)	16m 57s
unit_tests (latest, 8.1, multisite)	13m 21s
unit_tests (5.2, 7.2, singlesite)	12m 40s
unit_tests (5.2, 7.2, multisite)	12m 47s
	2h 26m 13s

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