Get rid of using enum in DomainPACVerifierCLIAdapter

Use fully-qualified enum names instead of `using enum` in
DomainPACVerfierAdapter::ProcessPACTypeOptions, becuase we use ancient
GCC-10 in CI, which does not dupport `using enum`.

Author: p-senichenkov

src/core/algorithms/pac/pac_verifier/domain_pac_verifier_cli_adapter.cpp

@@ -55,18 +55,18 @@ void DomainPACVerifierCLIAdapter::RegisterOptions() {
 }
 
 void DomainPACVerifierCLIAdapter::ProcessPACTypeOptions() {
-    using enum pac::model::DomainType::_enumerated;
+    using namespace pac::model;
 
     switch (domain_type_) {
-        case ball:
+        case DomainType::ball:
             domain_ = std::make_shared<Ball>(std::move(center_str_), radius_,
                                              std::move(leveling_coeffs_));
             break;
-        case parallelepiped:
+        case DomainType::parallelepiped:
             domain_ = std::make_shared<Parallelepiped>(std::move(first_str_), std::move(last_str_),
                                                        std::move(leveling_coeffs_));
             break;
-        case custom_domain:
+        case DomainType::custom_domain:
             __builtin_unreachable();
             break;
     }