v38 new locales to common and to ICU

Since Peter is out, reassigning to Mark

Expanded Recommendations

On the CLDR side:

1. We already have Sanskrit and Dogri on our list to move to common, as above.

2. Move Caddo and Guarani out of the CLDR target locales.

   1. They are already in seed, and were added as CLDR target locales when we thought we were going to do them.

3. Move Church Slavic, Prussian, Volapük from common to seed.

   1. Some marked as common have very low Basic and are not in ICU.

   2. Document in the Migration section

On the ICU side:

1. Add common CLDR that are missing in ICU (after above cases): Nigerian Pidgin, Sundanese, Sanskrit, Dogri, Manipuri, Maithili, Santali, Sindhi (Devanagari), Fulah (Adlam) go into ICU.

2. Remove very low-stat, non common locales: Low German

This is just for this release, and continuing past principles. We have two conjoined goals longer term:

1. Move the seed locales into the common directory.

2. Replace the "seed" vs "common" distinction by new coverage levels. That is, we would reorganize our coverage levels so that there is a category for "ready for ICU", where 100% qualifies and anything less doesn't. That would make the "decide what goes into ICU" pretty much turn-key. We may decide to retrofit Basic to fill this need, or may put something below Basic.

See also https://unicode-org.github.io/cldr-staging/charts/38/supplemental/locale_coverage.html

In Common/main: sa.xml, doi.xml

In Seed/main: cu.xml, prg.xml, vo.xml (Already in Seed cad.xml, gn.xml