Refactor: use rizin api for setting fcn calling convention

[PremadeS]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've updated the documentation with the relevant information (if needed)
• I've used AI tools to generate fully or partially these code changes and I'm sure the changes are not copyrighted by somebody else.

Detailed description

Use rz_analysis_function_set_cc() api to set function calling convention

Logic wise nothing changed, calling convention of the function can still be changed through:
Right Click -> Edit Function (Shift + P) -> Calling convention

A bit confused about the issue description (#3513) because cutter had the ability to change calling conventions long before the issue was opened.

Adding a new separate option in the menu itself, outside the Edit function dialog would be redundant in my opinion (if that what the user wants) , unless I've completely misinterpreted the issue

Test plan (required)

• Open Cutter with any binary
• Seek to any function
• Right Click -> Edit Function (or Shift + P)
• Change calling convention from the combo box
• Verify it changes successfully - can run afi in the console window

Closing issues

closes #3513
closes #3542

[notxvilka]

Yes, it doesn't resolve the main issue (rizinorg/rizin#5859) but improves the code a bit.