Also setup Java 21 for Windows jruby-head build #381

Job	Run time
ruby-versions / Generate Ruby versions	2s
windows-latest 2.7	2m 19s
windows-latest 3.0	1m 57s
windows-latest 3.1	1m 38s
windows-latest 3.2	1m 49s
windows-latest 3.3	1m 57s
windows-latest 3.4	3m 10s
windows-latest jruby-head	2m 12s
windows-latest mingw	1m 58s
windows-latest mswin	1m 45s
	18m 47s

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