Never change the underlying string encoding #459

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-24.04 2.7	38s
ubuntu-24.04 3.0	14s
ubuntu-24.04 3.1	10s
ubuntu-24.04 3.2	13s
ubuntu-24.04 3.3	17s
ubuntu-24.04 3.4	11s
ubuntu-24.04 debug	18s
ubuntu-24.04 head	15s
ubuntu-24.04 jruby-head	1m 0s
ubuntu-24.04 truffleruby-head	24s
ubuntu-22.04 2.7	37s
ubuntu-22.04 3.0	14s
ubuntu-22.04 3.1	12s
ubuntu-22.04 3.2	11s
ubuntu-22.04 3.3	19s
ubuntu-22.04 3.4	14s
ubuntu-22.04 debug	17s
ubuntu-22.04 head	21s
ubuntu-22.04 jruby-head	1m 5s
ubuntu-22.04 truffleruby-head	29s
	7m 41s

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