`ionex`: migrate from `interp2d` to `RegularGridInterpolator`

[yunjunz]

Description of proposed changes

• objects.ionex.get_ionex_value(): migrate from scipy.interpolate.interp2d() to scipy.interpolate.RegularGridInterpolator(), as the former has been removed since scipy-1.14.

• tests.objects.ionex.py: more print out msg

Reminders

• [x] Pass Pre-commit check (green)
• [x] Pass Codacy code review (green)
• [x] Pass Circle CI unit test (the circleci integration test is failing due to the known gdal issue https://github.com/insarlab/MintPy/issues/1220)
• [x] Pass local integration test
• [x] Make sure that your code follows our style. Use the other functions/files as a basis.
• [x] If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.