Update rubocop-performance requirement from ~> 1.20.0 to ~> 1.21.0 #2189

Job	Run time
lint	40s
spec (alpine-latest, 3.1)	33s
spec (alpine-latest, 3.2)	30s
spec (alpine-latest, 3.3)	38s
spec (macos-14, 3.1)	24s
spec (macos-14, 3.2)	23s
spec (macos-14, 3.3)	30s
spec (macos-14, jruby)	1m 8s
spec (macos-14, truffleruby)	1m 4s
spec (macos-14, truffleruby+graalvm)	3m 30s
spec (macos-latest, 3.1)	49s
spec (macos-latest, 3.2)	40s
spec (macos-latest, 3.3)	1m 4s
spec (macos-latest, jruby)	1m 57s
spec (macos-latest, truffleruby)	2m 27s
spec (macos-latest, truffleruby+graalvm)	4m 26s
spec (ubuntu-latest, 3.1)	16s
spec (ubuntu-latest, 3.2)	15s
spec (ubuntu-latest, 3.3)	33s
spec (ubuntu-latest, jruby)	1m 34s
spec (ubuntu-latest, truffleruby)	1m 10s
spec (ubuntu-latest, truffleruby+graalvm)	4m 36s
spec (windows-latest, 3.1)	1m 22s
spec (windows-latest, 3.2)	2m 47s
spec (windows-latest, 3.3)	3m 56s
spec (windows-latest, jruby)	2m 45s
release	0s
	39m 57s

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