Use GDALGetDataTypeSizeBytes since GDALGetDataTypeSize is deprecated

[remkos]

Description of proposed changes

This is to replace
GDALGetDataTypeSize(GDALGetRasterDataType(hBand)) / 8
by
GDALGetDataTypeSizeBytes(GDALGetRasterDataType(hBand))
since
GDALGetDataTypeSize (which was in bits) is marked as deprecated since GDAL v3.12.

This does not change any of the functionality.

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.