Add a note about chilled string support to 3.1.8 release note #586

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 3.0	12s
ubuntu-24.04 3.2	14s
ubuntu-24.04 3.1	18s
ubuntu-24.04 debug	21s
ubuntu-24.04 2.7	47s
ubuntu-24.04 truffleruby-head	33s
ubuntu-24.04 jruby-head	1m 8s
ubuntu-24.04 head	17s
ubuntu-22.04 3.1	17s
ubuntu-24.04 3.3	23s
ubuntu-22.04 2.7	49s
ubuntu-22.04 3.3	21s
ubuntu-22.04 3.2	13s
ubuntu-24.04 3.4	19s
ubuntu-22.04 3.4	19s
ubuntu-22.04 debug	27s
ubuntu-22.04 3.0	17s
ubuntu-22.04 head	12s
ubuntu-22.04 truffleruby-head	30s
ubuntu-22.04 jruby-head	1m 10s
	9m 12s

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