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GH-44898: [C++] Fix compilation error on GCC 8 (#44899)

### Rationale for this change

The `span` implementation featured a complicated SFINAE construct to enable the `span(range...)` constructor, which failed compiling on gcc 8.x.

### What changes are included in this PR?

Replace this complicated SFINAE with a much simpler one that avoids the recursive `span` template dependency while keeping the intended semantics.

### Are these changes tested?

Yes, by existing tests.

### Are there any user-facing changes?

No.
* GitHub Issue: #44898

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>