Add Ruby 4.0.0 to CI #2250

Job	Run time
RuboCop	46s
specs (ubuntu, 4.0.0)	1m 27s
specs (windows, 3.3.10)	3m 21s
specs (macos, 3.2.9)	1m 28s
specs (macos, 4.0.0)	1m 42s
specs (windows, 4.0.0)	3m 41s
specs (macos, 3.3.10)	1m 17s
specs (ubuntu, 3.2.9)	1m 36s
specs (windows, 3.2.8)	4m 15s
specs (ubuntu, 3.4.8)	1m 36s
specs (ubuntu, 4.0.0, --repeat 2)	2m 13s
specs (ubuntu, 4.0.0, --parser=parse.y)	1m 40s
specs (windows, 3.4.8)	3m 22s
specs (macos, 3.4.8)	1m 33s
specs (ubuntu, 3.3.10)	1m 32s
specs (ubuntu, 4.0.0, --enable-frozen-string-literal)	1m 30s
	32m 59s

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