ModGC

Move rb_gc_impl_ractor_cache_free to shutdown section #23

Job	Run time
check (mmtk, release, macos-latest)	10m 19s
check (default, ubuntu-latest)	7m 4s
check (mmtk, release, ubuntu-latest)	8m 35s
check (default, macos-latest)	6m 7s
	32m 5s

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