Unable to build with fmt 11

I see compilation errors when trying to build with GCC 14 or Clang 18 with fmt 11.0.2 that were not there before, like:

/usr/include/c++/14.2.1/type_traits:1110:25: error: invalid use of incomplete type ‘struct fmt::v11::formatter<Gaudi::Property<std::vector<int> >, char, void>’
 1110 |       = __bool_constant<__is_constructible(_Tp, _Args...)>;

These two lines are the ones with issues: https://gitlab.cern.ch/gaudi/Gaudi/-/blob/master/GaudiKernel/tests/src/test_PropertyFmt.cpp#L67-L68, for the test case with std::vector<int>.