PHPOffice library deprecated

[poonam279]

@contactashish13 A user reported this issue on wp.org.
https://wordpress.org/support/topic/phpexcel-deprecated/

The PHPOffice library that you are using, PHPExcel, was deprecated in 2015 and as of 2019 is permanently archived. The authors state that anyone using the library should upgrade to it’s direct successor. Just a heads up.

Deprecated: https://github.com/PHPOffice/PHPExcel
Successor: https://github.com/PHPOffice/PhpSpreadsheet

[contactashish13]

PR #369