[CDRIVER-4400] Rename references of "csfle" to "crypt_shared" (#1022)

* Rename references of "csfle" to "crypt_shared"
* use crypt_shared 6.0.0-rc8
* test with libmongocrypt master
* CDRIVER-4397 update fle2 find payloads
* update download-mongodb.sh to use 6.0.0-rc8
* add TODO comments to pin to 1.5.0-rc0
* fix check for crypt_shared availability
* remove unused compile definition

Co-authored-by: Kevin Albertson <kevin.albertson@mongodb.com>
Co-authored-by: Ezra Chung <88335979+eramongodb@users.noreply.github.com>