Use Java 21 for released JRuby as well #456

Job	Run time
ruby-versions / Generate Ruby versions	1s
ubuntu-latest 2.4	1m 2s
windows-latest 2.4	3m 52s
ubuntu-latest 2.5	19s
windows-latest 2.5	2m 46s
ubuntu-latest 2.6	1m 19s
macos-latest 2.6	1m 24s
windows-latest 2.6	3m 16s
ubuntu-latest 2.7	1m 9s
macos-latest 2.7	1m 15s
windows-latest 2.7	4m 19s
ubuntu-latest 3.0	13s
macos-latest 3.0	16s
windows-latest 3.0	1m 44s
ubuntu-latest 3.1	11s
macos-latest 3.1	21s
windows-latest 3.1	2m 8s
ubuntu-latest 3.2	14s
macos-latest 3.2	11s
windows-latest 3.2	1m 46s
ubuntu-latest 3.3	13s
macos-latest 3.3	13s
windows-latest 3.3	2m 8s
ubuntu-latest 3.4	11s
macos-latest 3.4	15s
windows-latest 3.4	2m 46s
ubuntu-latest head	15s
macos-latest head	16s
ubuntu-latest jruby	1m 55s
macos-latest jruby	1m 11s
windows-latest jruby	5m 17s
ubuntu-latest jruby-head	1m 59s
macos-latest jruby-head	1m 17s
windows-latest jruby-head	4m 18s
ubuntu-latest truffleruby-head	37s
macos-latest truffleruby-head	37s
windows-latest mingw	1m 17s
windows-latest mswin	2m 3s
	54m 34s

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