[C++] Deprecate Scalar::CastTo

[bkietz]

Describe the enhancement requested

#35560 removed uses of Scalar::CastTo in favor of compute::Cast. Now that the codebase doesn't use it, we can mark it deprecated and (eventually) remove it https://github.com/apache/arrow/blob/main/cpp/src/arrow/scalar.h#L99-L100

Component(s)

C++

[bkietz]

@llama90

[llama90]

Progress Update

If we remove the legacy CastTo, we must also update the ToString function to call the new Cast function.

I suspect that the following unit tests are failing due to a lack of functionality in the new Cast function specifically for ToString.

• arrow-misc-test
• arrow-scalar-test
• arrow-compute-internals-test
• arrow-compute-expression-test
• arrow-compute-scalar-if-else-test
• arrow-compute-scalar-math-test
• arrow-compute-vector-test
• arrow-compute-aggregate-test
• arrow-acero-plan-test
• arrow-acero-hash-aggregate-test
• arrow-dataset-dataset-test
• arrow-dataset-file-ipc-test
• arrow-dataset-file-test
• arrow-dataset-partition-test
• arrow-dataset-scanner-test
• arrow-dataset-file-csv-test
• arrow-dataset-file-json-test
• arrow-dataset-file-orc-test
• arrow-dataset-file-parquet-test
• arrow-dataset-file-parquet-encryption-test
• arrow-flight-sql-test
• arrow-substrait-substrait-test
• gandiva-projector-test