cetl::pf20::span should accept pointers as its template T type.

[serges147]

At libcyphal currently there is such todo:

    CETL_NODISCARD static inline Expected<UniquePtr<IUdpTransport>, FactoryError> make(
        cetl::pmr::memory_resource&  memory,
        IMultiplexer&                mux,
        const std::array<IMedia*, 3> media,  // TODO: replace with `cetl::span<IMedia*>`
        const cetl::optional<NodeId> local_node_id);

Making it as below works only on c++ 20 (where there is no our span polyfil):

    ...
        const cetl::span<IMedia*>    media,
    ...

, but it fails on c++14 & 17 with following compiler error:

In file included from /Users/sergei/Develop/git/OpenCyphal-Garage/libcyphal/external/cetl/include/cetl/pf20/cetlpf.hpp:63:
/Users/sergei/Develop/git/OpenCyphal-Garage/libcyphal/external/cetl/include/cetl/pf20/span.hpp:492:5: error: static assertion failed due to requirement '!std::is_pointer<libcyphal::transport::can::IMedia *>::value': Can not be a pointer type
    static_assert(not std::is_pointer<T>::value, "Can not be a pointer type");
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/sergei/Develop/git/OpenCyphal-Garage/libcyphal/include/libcyphal/transport/can/transport.hpp:97:34: note: in instantiation of template class 'cetl::pf20::span<libcyphal::transport::can::IMedia *>' requested here
    const cetl::span<IMedia*>    media,  // TODO: replace with `cetl::span<IMedia*>`
...