Use prism_gravity on gradient-boosted tests

[santisoler]

Replace the old point_mass_gravity for the point_gravity function in gradient-boosted equivalent sources tests. The old function name was still there because of the simultaneous development of the new feature with the renaming of the function.

Reminders:

• [ ] Run make format and make check to make sure the code follows the style guide.
• [ ] Add tests for new features or tests that would have caught the bug that you're fixing.
• [ ] Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• [ ] Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• [ ] If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• [ ] Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[santisoler]

I'm merging this. No need to modify the changelog in #290 since this PR is fixing some issues on the renaming of the prism_gravity function, nothing new.

For some reason GitHub is not reporting the codecov outputs, even if they run and pass on their website. So I'm merging this by force.