MINOR: [C++] Avoid using deprecated RecordBatchReader::ReadAll
MINOR: [C++] Avoid using deprecated RecordBatchReader::ReadAll

Fix this compilation error:
```
/home/antoine/arrow/dev/cpp/src/arrow/dataset/file_orc_test.cc:45:27: error: 'ReadAll' is deprecated: Deprecated in 8.0.0. Use ToTable instead. [-Werror,-Wdeprecated-declarations]
    RETURN_NOT_OK(reader->ReadAll(&table));
                          ^
```

Closes #12708 from pitrou/minor-cpp-deprecation

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>

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