dev: Update Wasm GC stability docs (#13406)

I think we can consider it fuzzed between the new GC accessors fuzzer,
wasm-smith's Wasm GC support, and the ongoing GC ops fuzzer work. Additionally,
when we accidentally introduced GC-related bugs recently (e.g. when splitting GC
heap tunables out from the existing memory tunables) the fuzzers quickly and
successfully found these bugs.