macos

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

Job	Run time
ruby-versions / Generate Ruby versions	4s
macos-13 3.0	22s
macos-13 3.1	29s
macos-13 3.2	31s
macos-13 3.4	27s
macos-13 2.7	1m 20s
macos-13 3.3	39s
macos-13 debug	30s
macos-13 head	24s
macos-14 2.7	33s
macos-14 3.4	15s
macos-14 3.2	17s
macos-14 3.1	21s
macos-13 jruby-head	2m 0s
macos-14 debug	26s
macos-14 3.0	16s
macos-14 3.3	17s
macos-14 head	19s
macos-14 jruby-head	43s
	10m 13s

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