libsystemd: use C23 explicit enum types where available

libsystemd: use C23 explicit enum types where available

In C23 we can explicitly choose the integer type for an enum. Let's do
so to make our requirements for 64bit integers explicitly. Previously,
we'd rely on a GNU extension that would size the enum to 64bit if at
least one value outside the 32bit range is in the enum. Let's keep that
too, for compat with older compilers.

(Also, add the support for older compilers to the definition of
sd_json_dispatch_flags_t, where it was forgotten so far)