In the cat subcommand, replace the usage of the deprecated PdfMerger class from pypdf

[Lucas-C]

PdfMerger is used in the in the cat subcommand: https://github.com/py-pdf/pdfly/blob/main/pdfly/cat.py#L51

But this class is deprecated in pypdf since version 3.9.1: https://github.com/py-pdf/pypdf/releases/tag/3.9.1

We should replace it by combining PdfReader, its merge_page() method & PdfWriter from pypdf.
An example of code using those can be found there: https://py-pdf.github.io/fpdf2/CombineWithPypdf.html

Extra unit tests could be added in the process in https://github.com/py-pdf/pdfly/blob/main/tests/test_cat.py
in order to ensure that pdfly cat still works similarly as it currently does.