Changes for 8.1.0_RC2 (DOI-USGS#5387)

* Fix dependency formatting in meta.yaml

* Pin gsl, update docs

* Revert pin

* Pin gsl to 2.6

* Add 8.1.0_RC1 as separate code entry

* Update the urls for 8.0 entry

* Turn on python bindings to be installed during build

* Add back kakadu build