Update expiration date for gpg release signing key to 2028-07-17. Was due to expire 2023-07-10. Also updated documentation with the new public key. Added gpg directions on how to update the signing key to RELEASE.txt.

#!/usr/bin/env python
from __future__ import print_function
import zlib, base64, sys
print(base64.encodestring(zlib.compress(sys.stdin.read())))