Add 3.1.8 release note (#176) #576

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.2	23s
ubuntu-24.04 3.0	13s
ubuntu-24.04 3.3	23s
ubuntu-24.04 2.7	46s
ubuntu-24.04 3.1	18s
ubuntu-24.04 3.4	17s
ubuntu-24.04 head	14s
ubuntu-22.04 3.2	20s
ubuntu-24.04 jruby-head	1m 16s
ubuntu-24.04 debug	31s
ubuntu-22.04 2.7	43s
ubuntu-22.04 3.0	18s
ubuntu-22.04 3.1	13s
ubuntu-22.04 3.4	16s
ubuntu-22.04 head	15s
ubuntu-22.04 3.3	19s
ubuntu-22.04 jruby-head	1m 9s
ubuntu-24.04 truffleruby-head	36s
ubuntu-22.04 truffleruby-head	36s
ubuntu-22.04 debug	30s
	9m 40s

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