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