Migrate to stac-utils org

[TomAugspurger]

With three contributors now (thanks @gadomski, @m-mohr) it's probably time to move this out of my personal GitHub org.

I plan to migrate this to the stac-utils org sometime in the next couple days, unless I hear any objections.

[m-mohr]

Fine for me. The only reason I could think of is that there seems to be a lot of MS PC specific code in there which you may depend on? And then it might be better to keep some control?

[TomAugspurger]

I think https://github.com/TomAugspurger/stac-geoparquet/blob/main/stac_geoparquet/pc_runner.py is too Planetary Computer-specific and should be removed. I have a better home for that now in microsoft/planetary-computer-tasks#232 anyway, though it'll take a bit of time for me to get that updated.

https://github.com/TomAugspurger/stac-geoparquet/blob/main/stac_geoparquet/pgstac_reader.py is a mixture of things that are generic to pgstac and Planetary Comptuer. With a small refactor, I can get a pretty good split.

[TomAugspurger]

Migrated to https://github.com/stac-utils/stac-geoparquet.

I opened #16 for cleaning stuff up.