Also setup Java 21 for Windows jruby-head build #440

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-24.04 2.7	50s
ubuntu-24.04 3.0	20s
ubuntu-24.04 3.1	20s
ubuntu-24.04 3.2	25s
ubuntu-24.04 3.3	23s
ubuntu-24.04 3.4	20s
ubuntu-24.04 debug	26s
ubuntu-24.04 head	19s
ubuntu-24.04 jruby-head	1m 17s
ubuntu-24.04 truffleruby-head	43s
ubuntu-22.04 2.7	43s
ubuntu-22.04 3.0	18s
ubuntu-22.04 3.1	25s
ubuntu-22.04 3.2	27s
ubuntu-22.04 3.3	20s
ubuntu-22.04 3.4	20s
ubuntu-22.04 debug	32s
ubuntu-22.04 head	21s
ubuntu-22.04 jruby-head	1m 19s
ubuntu-22.04 truffleruby-head	40s
	10m 50s

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