bump version to 5.1.6 (#1213) #1511

Job	Run time
checkstyle	20s
e2e_tests (latest, 7.2)	15m 9s
unit_tests (latest, 7.2, singlesite)	14m 3s
e2e_tests (latest, 8.1)	15m 19s
e2e_tests (trunk, 7.2)	15m 16s
unit_tests (latest, 7.2, multisite)	11m 47s
e2e_tests (trunk, 8.1)	15m 7s
unit_tests (latest, 8.1, singlesite)	13m 37s
unit_tests (latest, 8.1, multisite)	14m 40s
unit_tests (5.2, 7.2, singlesite)	11m 54s
unit_tests (5.2, 7.2, multisite)	12m 58s
	2h 20m 10s

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