Update the GC tiered level from B to C.

[lum1n0us]

Several known limitations have been exposed, and a few might impact the estimation when trying to integrate that feature into a product.

• How to handle ref.null 0? #4542
• Cannot correctly release a field (anyref) in a struct or an array in destroy_init_expr_data_recursive() #4816
• fix nested struct init value error in gc aot #4176

[TianlongLiang]

@no1wudi do you have any thoughts on this? @lum1n0us and I don't have too much bandwidth working on syncing the implementation with the newest GC proposal. I wonder whether you will be interested/needed the newest GC proposal. If you are not interested either, I think it's safe for us to update the GC proposal to tier C(won't affected the fact the maintenance of existing parts) as this PR does.