Switch to gcc-13 for C++23 tests

[phprus]

But there are some build errors (potential null pointer dereference [-Werror=null-dereference]):

https://github.com/phprus/fmt/actions/runs/10708260895/job/29690153702

@vitaut

[vitaut]

Do you have an idea what pointer this warning refers to? __location?

/usr/include/c++/13/bits/stl_construct.h:97:14: error: potential null pointer dereference [-Werror=null-dereference]
   97 |     { return ::new((void*)__location) _Tp(std::forward<_Args>(__args)...); }
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[phprus]

This is gcc bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108860

[phprus]

Errors from GoogleTest: https://github.com/fmtlib/fmt/actions/runs/10718554817/job/29720727961?pr=4155

[phprus]

@vitaut
All errors is fixed.

[vitaut]

Thank you!