Update minimum ruby version to 3.0. #248

Job	Run time
test-yjit (ubuntu, 3.1)	56s
test-yjit (ubuntu, 3.1)	56s
test-yjit (ubuntu, 3.2)	49s
test-yjit (ubuntu, 3.2)	49s
test (ubuntu, 3.0)	53s
test (ubuntu, 3.0)	53s
test-yjit (ubuntu, 3.3)	47s
test-yjit (ubuntu, 3.3)	47s
test (ubuntu, 3.1)	52s
test (ubuntu, 3.1)	52s
test-yjit (ubuntu, head)	50s
test-yjit (ubuntu, head)	50s
test (ubuntu, 3.2)	56s
test (ubuntu, 3.2)	56s
test (ubuntu, 3.3)	50s
test (ubuntu, 3.3)	50s
test-yjit (macos, 3.1)	1m 29s
test-yjit (macos, 3.1)	1m 29s
test (ubuntu, head)	48s
test (ubuntu, head)	48s
test (ubuntu, jruby)	1m 31s
test (ubuntu, jruby)	1m 31s
test-yjit (macos, 3.2)	1m 18s
test-yjit (macos, 3.2)	1m 18s
test (ubuntu, jruby-head)	1m 34s
test (ubuntu, jruby-head)	1m 20s
test-yjit (macos, 3.3)	53s
test-yjit (macos, 3.3)	53s
test (ubuntu, truffleruby)	1m 31s
test (ubuntu, truffleruby)	1m 31s
test-yjit (macos, head)	1m 17s
test-yjit (macos, head)	1m 17s
test (ubuntu, truffleruby-head)	1m 39s
test (ubuntu, truffleruby-head)	1m 39s
test (macos, 3.0)	1m 6s
test (macos, 3.0)	1m 6s
test (windows, 3.0)	1m 24s
test (windows, 3.0)	1m 24s
test (windows, ucrt)	55s
test (windows, ucrt)	55s
test (windows, mswin)	1m 5s
test (windows, mswin)	1m 5s
finish	5s
finish	5s
	46m 42s

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