util: do not use variable length array

vla (variable length array) is an extension in GCC and Clang. and
it is not part of the C++ standard.

so let's avoid using it if possible, for better standard compliant.
it's also more consistent with other places where we calculate the size
of an array of T in the same source file.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Closes scylladb/scylladb#16084

utils/managed_vector.hh

@@ -34,7 +34,7 @@ private:
             _backref->_data = _data;
         }
         size_t storage_size() const noexcept {
-            return sizeof(*this) + sizeof(T[_backref->_capacity]);
+            return sizeof(*this) + sizeof(T) * _backref->_capacity;
         }
     };
     union maybe_constructed {