Fix significant subdomain detection for custom top-level domains in cutToFirstSignificantSubdomainCustom() and firstSignificantSubdomainCustom() functions

Fix cutToFirstSignificantSubdomainCustom()/firstSignificantSubdomainCustom() returning wrong result for 3+ level domains present in custom top-level domain list. For input domains matching these custom top-level domains, the third-level domain was considered to be the first significant one. This is now fixed. This change may introduce incompatibility if the function is used in e.g. the sharding key. #21946 (Azat Khuzhin).