Fix ruby version guard for Fiber.[] #1868

Job	Run time
RuboCop	30s
specs (ubuntu, 3.0.6)	1m 30s
specs (ubuntu, 3.1.4)	1m 33s
specs (ubuntu, 3.2.2)	1m 31s
specs (ubuntu, 3.3.0)	1m 24s
specs (macos, 3.0.6)	1m 33s
specs (macos, 3.1.4)	1m 55s
specs (macos, 3.2.2)	1m 28s
specs (macos, 3.3.0)	1m 13s
specs (windows, 3.0.6)	3m 24s
specs (windows, 3.1.4)	4m 19s
specs (windows, 3.2.2)	4m 40s
specs (windows, 3.3.0)	6m 25s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 29s
	32m 54s

Provide feedback