Add major GR banks to el_GR bank provider

[bon12-gr]

What does this change

Added a banks tuple to the el_GR bank provider containing 13 GR banks, including systemic banks (e.g. Alpha Bank) and cooperative banks (e.g. Cooperative Bank of Chania). I only added banks that have a head office in Greece, and not branches of foreign-based banks. The information is sourced from the Bank of Greece (english, greek).

What was wrong

Previously, a call to the bank() method when using the el_GR locale would raise an attributeError, because the provider did not have a banks tuple.

How this fixes it

Added the missing banks tuple to the el_GR/__init__.py file, enabling the bank() method to work correctly.

Fixes #2305

A few comments

This is my first contribution to open source. I apologise if I missed a contributing standard. The linter technology is new to me, and to my understanding I cannot run make linter, because I use a Windows machine. I used gen-AI (see section below) to run a list of "equal" commands based on Makefile. If I can do anything else please let me know.

AI usage

I should clarify that I used gen-AI (specifically Google Gemini) to:

• correct my PR message so that I use proper technical terminology.
• run style checks on my code.

Checklist

• I have read the documentation about CONTRIBUTING
• I have read the documentation about Coding style
• I have run make lint