refactor: Prefer Quantity.backend over gt4py_backend

[romanc]

Description

gt4py_backend is about to be deprecated and renamed to backend, see NOAA-GFDL/NDSL#312.

How Has This Been Tested?

Assumed to be covered by existing tests.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included: N/A
• Targeted model if this changed was triggered by a model need/shortcoming: N/A

[romanc]

blocked by the combinations of NDSL and pyFV3 versions of pace tests

[romanc]

Superseded by #109.