refactor: _min_cppstd replaced with an inlined version

conanfile.py

@@ -74,10 +74,6 @@ class MPUnitsConan(ConanFile):
     package_type = "header-library"
     no_copy_source = True
 
-    @property
-    def _min_cppstd(self):
-        return "20"
-
     @property
     def _minimum_compilers_version(self):
         return {
@@ -123,7 +119,7 @@ class MPUnitsConan(ConanFile):
                 self.test_requires("wg21-linear_algebra/0.7.3")
 
     def validate(self):
-        check_min_cppstd(self, self._min_cppstd)
+        check_min_cppstd(self, "20")
 
         def loose_lt_semver(v1, v2):
             lv1 = [int(v) for v in v1.split(".")]