Downgrade PHPUnit to 9.6.12 #7020

Job	Run time
Parse.ly: 3.1 filter_enabled	8m 43s
Parse.ly: 3.1 filter_disabled	10m 29s
Parse.ly: 3.1 option_enabled	8m 51s
Parse.ly: 3.1 option_disabled	8m 31s
Parse.ly: 3.1 filter_and_option_enabled	10m 19s
Parse.ly: 3.1 filter_and_option_disabled	9m 5s
Parse.ly: filter_enabled	10m 1s
Parse.ly: filter_disabled	8m 37s
Parse.ly: option_enabled	8m 51s
Parse.ly: option_disabled	10m 24s
Parse.ly: filter_and_option_enabled	10m 28s
Parse.ly: filter_and_option_disabled	8m 36s
	1h 52m 55s

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