Bump sinon from 16.1.3 to 21.0.0 in the tests-qunit group #568

Job	Run time
Determine Matrix	3s
Multisite Default / Test current	13m 7s
Multisite Memcached / Test current	12m 54s
Single Site Memcached / Test current	12m 49s
Single Site Default / Test current	12m 40s
Compare / Multisite Default	11s
Compare / Multisite Memcached	11s
Slack Notifications	0s
Compare / Single Site Default	11s
Compare / Single Site Memcached	17s
Failed workflow tasks	0s
	52m 23s

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