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GH-40366: [C++] Remove const qualifier from Buffer::mutable_span_as (#40367)



### Rationale for this change

The const qualifier on `Buffer:mutable_span_as` prevents it from calling the non-const `Buffer::mutable_data_as`

### What changes are included in this PR?

See issue title

### Are these changes tested?

No, I believe this is a simple oversight 

### Are there any user-facing changes?

Yes, `Buffer::mutable_span_as` loses it's const qualifier

* GitHub Issue: #40366

Authored-by: Simon Perkins <simon.perkins@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>