Prevent make_span from working with vectors and arrays of pointers

[are-you-tilted-already]

The goal is to fix the problem with a confusing behaviour of flatbuffers::make_span with flatbuffers::Vector<(const)? T*>.
Consider the following flatbuffers schema:

// example.fbs
struct ExampleStruct {
  id: uint64;
}

table ExampleStructs {
  values: [ExampleStruct];
}

and some code working with it:

#include "example.fbs.h"

void ProcessStruct(const ExampleStruct& value);

void ProcessStructs(const flatbuffers::span<const ExampleStruct*> values) {
  for (const ExampleStruct* const value : values) {
    ProcessStruct(*value);
  }
}

const ExampleStructs* const table = ...;

// UB and most likely SIGSEGV, because the correct way to read is the one that flatbuffers::IndirectHelper<const T*> uses
// https://github.com/google/flatbuffers/blob/master/include/flatbuffers/buffer.h#L168
ProcessStructs(flatbuffers::make_span(table->values());

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[are-you-tilted-already]

Some people (including me, to be honest) use spans almost whenever they could, so it's better to make flatbuffers::Vector<const T*> to flatbuffers::Span<const T*> conversion impossible.

[are-you-tilted-already]

@aardappel @dbaileychess what do you think of it?

[aardappel]

I agree such an illegal conversion should not be possible, so disabling it is better than nothing.

Ideally it be converted to flatbuffers::Span<const T> instead, if on little endian? do we want to make that possible somehow?

And looks like CI wants some parentheses to clarify precedence.