Use final ref for projjson

[gadomski]

The current ref, https://proj.org/schemas/v0.7/projjson.schema.json, is actually a redirect to https://proj.org/en/latest/schemas/v0.7/projjson.schema.json:

$ curl -s -D - https://proj.org/schemas/v0.7/projjson.schema.json -o /dev/null | head -n5
HTTP/2 302 
date: Thu, 26 Sep 2024 12:10:21 GMT
content-type: text/html; charset=utf-8
content-length: 0
location: https://proj.org/en/latest/schemas/v0.7/projjson.schema.json

This isn't wrong per-se, but does cause an extra request when validating which is slightly annoying. This was discovered because that redirect is causing breakages in rare situations, e.g. stac-utils/pystac#1409.