Add 3.1.9 release note #536

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 2.7	1m 34s
windows-latest 3.1	1m 15s
windows-latest 3.2	1m 3s
windows-latest 3.0	1m 4s
windows-latest 3.3	1m 6s
windows-latest 3.4	1m 50s
windows-latest jruby-head	1m 43s
windows-latest mingw	57s
windows-latest mswin	1m 25s
	12m 1s

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