Upgrade the lowest Ruby version to 3.1 #1901

Job	Run time
RuboCop	35s
specs (ubuntu, 3.1.6)	1m 32s
specs (ubuntu, 3.2.7)	3s
specs (ubuntu, 3.3.7)	1m 30s
specs (ubuntu, 3.4.1)	1m 37s
specs (macos, 3.1.6)	1m 32s
specs (macos, 3.2.7)	6s
specs (macos, 3.3.7)	1m 31s
specs (macos, 3.4.1)	1m 16s
specs (windows, 3.1.6)	4m 35s
specs (windows, 3.2.7)	12s
specs (windows, 3.3.7)	4m 9s
specs (windows, 3.4.1)	5m 8s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 27s
	25m 13s

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