Use previous RDoc on JRuby #620

Job	Run time
ruby-versions / Generate Ruby versions	6s
windows-latest 3.4	1m 20s
windows-latest 4.0	56s
windows-latest 2.7	1m 49s
windows-latest 3.0	1m 15s
windows-latest jruby-head	2m 5s
windows-latest 3.1	1m 6s
windows-latest 3.3	1m 44s
windows-latest 3.2	1m 6s
windows-latest mingw	59s
windows-latest mswin	1m 8s
	13m 34s

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