pkg: Move FreeBSD-base to pkg.freebsd.org

Rather than fetching packages directly from the CDN which currently
backs pkgbase.freebsd.org, requests will go to pkg.freebsd.org mirrors
and be 302ed to the correct servers.  This adds ~70 seconds to the
process of installing or upgrading a pkgbase system; it also orphans
systems with 15.0-{PRERELEASE,ALPHA*,BETA*} installed since they are
expecting to see pkgbase files signed with the pkg keys, not the new
pkgbase signing keys.

Reviewed by:	dch, philip
MFC after:	immediately (for 15.0-RELEASE)
With hat:	re
Requested by:	clusteradm, core
Differential Revision:	https://reviews.freebsd.org/D53964

usr.sbin/pkg/FreeBSD.conf.quarterly-release

@@ -24,7 +24,7 @@ FreeBSD-ports-kmods: {
   enabled: yes
 }
 FreeBSD-base: {
-  url: "pkg+https://pkgbase.FreeBSD.org/${ABI}/base_release_${VERSION_MINOR}",
+  url: "pkg+https://pkg.FreeBSD.org/${ABI}/base_release_${VERSION_MINOR}",
   mirror_type: "srv",
   signature_type: "fingerprints",
   fingerprints: "/usr/share/keys/pkgbase-${VERSION_MAJOR}",