Use 3.1.2 #333

Job	Run time
ruby-versions / Generate Ruby versions	1s
windows-latest 2.7	2m 59s
windows-latest 3.0	1m 9s
windows-latest 3.1	1m 8s
windows-latest 3.2	2m 25s
windows-latest 3.3	2m 38s
windows-latest mingw	1m 0s
windows-latest mswin	1m 19s
	12m 39s

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