fix build errors

Author: DeagleGross

src/Http/Owin/src/DictionaryStringArrayWrapper.cs

@@ -108,7 +108,7 @@ public bool MoveNext()
 
         public KeyValuePair<string, string[]> Current => _current;
 
-        object? IEnumerator.Current => Current;
+        object IEnumerator.Current => Current;
 
         void IEnumerator.Reset() => throw new NotImplementedException();
     }

src/Http/Owin/src/DictionaryStringValuesWrapper.cs

@@ -153,7 +153,7 @@ public bool MoveNext()
 
         public KeyValuePair<string, StringValues> Current => _current;
 
-        object? IEnumerator.Current => Current;
+        object IEnumerator.Current => Current;
 
         void IEnumerator.Reset() => throw new NotImplementedException();
     }