Upgrade the lowest Ruby version to 3.1 #1905

Job	Run time
RuboCop	31s
specs (ubuntu, 3.1.6)	1m 37s
specs (ubuntu, 3.2.6)	1m 28s
specs (ubuntu, 3.3.7)	1m 36s
specs (ubuntu, 3.4.1)	1m 37s
specs (macos, 3.1.6)	1m 27s
specs (macos, 3.2.6)	1m 19s
specs (macos, 3.3.7)	1m 22s
specs (macos, 3.4.1)	1m 21s
specs (windows, 3.1.6)	4m 13s
specs (windows, 3.2.6)	4m 9s
specs (windows, 3.3.7)	4m 8s
specs (windows, 3.4.1)	5m 20s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 24s
	31m 32s

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