[GR-44714] [GR-49652] Move darwin-amd64 CI from Mojave to Big Sur #4269

Job	Run time
ruby/spec on CRuby 3.0	3m 22s
ruby/spec on CRuby 3.1	3m 33s
build native	14m 48s
ruby/spec on CRuby 3.2	2m 56s
lint	14m 2s
build jvm	6m 29s
specs :truffle	9m 51s
specs :language :core	12m 39s
test integration	7m 35s
fast specs	2m 12s
specs :library :cext :security :command_line :tracepoint	9m 47s
test native	3m 16s
run MRI tests on native	16m 14s
	1h 46m 44s

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