Workbook::getSheetName() returning null due to sheet name truncation to 31 characters.

In 5.x versions, while creating a sheet, it was decided to truncate its name:

`String sn = (sheetname.length() > 31) ? sheetname.substring(0, 31) : sheetname;`

But as a consequence, trying to get the sheet using the same original name results in null.

```import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.hssf.usermodel.HSSFWorkbook;

public class ApachePOISheetNameTest {
private static final String LONG_SHEET_NAME = "ThisSheetNameIsMoreThan31Characters";

public static void main(String[] args) throws Exception {
try (Workbook workbook = new HSSFWorkbook()) {
workbook.createSheet(LONG_SHEET_NAME);
Sheet sheet = workbook.getSheet(LONG_SHEET_NAME);
System.out.println(sheet); // null
sheet = workbook.getSheet(LONG_SHEET_NAME.substring(0, 31));
System.out.println(sheet); // works
}
}

}
```

Start by tracing the Workbook.createSheet(String) and Workbook.getSheet(String) entry points involved in the example, including the 31-character truncation described in the issue. Reproduce the HSSFWorkbook case and add coverage showing the expected result when the original long sheet name is used for lookup.

Written by the indexing model from the issue text.