Use astropy-supplied data files with freeze_iers()

[weaverba137]

Astropy now supplies frozen versions of Earth rotation and leap second tables (astropy-iers-data), which are automatically installed when Astropy is installed.

At the same time, the older, frozen version of the Earth rotation table previously installed by desiutil.iers.update_iers() is too old to be used with Astropy 7, since the format of the data has changed, and astropy 7 expects the new format.

For future desiutil 4.0, we propose to abandon the separate table internal to desiutil and use Astropy's tables. However, freeze_iers() is probably still needed at some level to turn off any possible downloads of updates.

The current iers_frozen.ecsv was added 6 April 2020. Although it was modified in 2021, that modification was to the metadata of the file, not the actual data table.

In addition, some operational testing is needed at least to cover these scenarios:

1. If a user doesn't execute freeze_iers() triggers a download of IERS data, we assume that version lives in the user's Astropy cache directory. We want to ensure that this cached version will be ignored when freeze_iers() is executed. In other words, freeze_iers() should guarantee that only the official files in astropy-iers-data are ever used.
2. Fiberassign reproducibility: we need to thoroughly test fiberassign reproducibility with newer IERS tables. Given that the desiutil IERS data table was updated in April 2020, that basically means the entire survey. We will need to bring in and inform the survey ops team for this.