migrate Ghidra backend from Ghidrathon to PyGhidra

[williballenthin]

Ghidra 11.3 was recently released with the built-in PyGhidra Python API bindings: https://github.com/NationalSecurityAgency/ghidra/blob/Ghidra_11.3_build/Ghidra/Configurations/Public_Release/src/global/docs/WhatsNew.md#pyghidra

We should consider migrating capa's Ghidra backend from Ghidrathon, which is a third-party binding that requires installation and maintenance, to PyGhidra. We'd want to ensure the performance remains acceptable (PyGhidra relies on JPype). Otherwise, I expect the migration to be fairly straightforward, particularly because of our unit tests.