Changed RDA urls to rebranded GDEX urls

[rpconroy]

NCAR's Research Data Archive (RDA) has rebranded as the Geoscience Data Exchange (GDEX). This PR updates the GFS model's
references to match.

Changes

• src/herbie/models/gfs.py
  • Changed the archive URLs from data.rda.ucar.edu to data.gdex.ucar.edu.
  • Changed the dataset link from rda.ucar.edu/datasets/d084001/ to gdex.ucar.edu/datasets/d084001/.
  • Renamed the source key ncar_rda to ncar_gdex in both SOURCES entries.
  • Updated the docstring and the source label to "NCAR Geoscience Data Exchange (GDEX)".
• docs/gallery/noaa_models/gfs.ipynb: Updated the example output URLs to the new GDEX host.
• unique_gfs.ipynb: Updated a commented-out URL to the new host.

Notes

• Breaking change: the ncar_rda source key was renamed to ncar_gdex. Anyone passing priority="ncar_rda" will need to
  switch to "ncar_gdex". If you want to avoid breaking them, you could keep ncar_rda as an alias.
• Stale output in the gallery notebook: the gfs.ipynb change edited the saved output text by hand. The output still shows
  the ncar_rda key, so re-running the notebook would show ncar_gdex.

[blaylockbk]

Thanks for the PR!
I've been a bit busy to do a full review, but wanted you know I see this and am tracking it.

I do like the idea of an alias and using a future warning to alert users it changed.