Update README.md and remove Ruby 3.1 mentions #1958

Job	Run time
specs (ubuntu, 3.2.7)	1m 27s
specs (ubuntu, 3.3.7)	1m 33s
RuboCop	51s
specs (ubuntu, 3.4.2)	1m 35s
specs (macos, 3.2.7)	1m 30s
specs (macos, 3.3.7)	1m 23s
specs (macos, 3.4.2)	1m 11s
specs (windows, 3.2.7)	4m 6s
specs (windows, 3.3.7)	4m 46s
specs (windows, 3.4.2)	5m 14s
specs (ubuntu, 3.4.2, --enable-frozen-string-literal)	1m 34s
	25m 10s

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