refactor: use new constructor of QuantityFactory

[romanc]

Description

Prefer the new constructor of QuantityFactory over the deprecated call to QuantityFactory.from_backend(...). This removes a bunch of deprecation warnings in tests.

See NOAA-GFDL/NDSL#228 for context.

/cc @FlorianDeconinck

How Has This Been Tested?

Search and manual replace of from_backend within pySHiELD. CI is still green.

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 change was triggered by a model need/shortcoming: N/A

[romanc]

@oelbert I'd like to move on this PR. It is blocking NOAA-GFDL/NDSL#356, which we scheduled for the next NDSL release. A similar PR in PyFV3 was approved and is ready to merge. Is there anything I can do from my side to get traction on this simple refactor?