Align MaxPool op attribute 'rounding_type' to spec.

Attribute name should be in lower case.

inference-engine/tests/functional/plugin/cpu/shared_tests_instances/skip_tests_config.cpp

@@ -10,7 +10,7 @@
 std::vector<std::string> disabledTestPatterns() {
     return {
         // TODO: Issue 26264
-        R"(.*(MaxPool|AvgPool).*S\(1\.2\).*Rounding=CEIL.*)",
+        R"(.*(MaxPool|AvgPool).*S\(1\.2\).*Rounding=ceil.*)",
         // TODO: Issue 31841
         R"(.*(QuantGroupConvBackpropData3D).*)",
         // TODO: Issue 31843

ngraph/core/src/op/util/attr_types.cpp

@@ -66,7 +66,7 @@ namespace ngraph
     {
         static auto enum_names = EnumNames<op::RoundingType>(
             "op::RoundingType",
-            {{"FLOOR", op::RoundingType::FLOOR}, {"CEIL", op::RoundingType::CEIL}});
+            {{"floor", op::RoundingType::FLOOR}, {"ceil", op::RoundingType::CEIL}});
         return enum_names;
     }