Don't use deprecated `Cop.registry` in specs #108

Job	Run time
JRuby 9.4	2m 41s
Ruby 2.7	1m 5s
Check documentation syntax	33s
Ruby 3.0	47s
Check the oldest supported RuboCop version	1m 1s
Ruby 3.1	52s
Prism	53s
Ruby 3.2	48s
Ruby 3.3	47s
Ruby head	44s
	10m 11s

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