more release checklist updates

signers@isc.org is no longer a valid address, I get a 550 5.1.1 SMTP error when I try to send to it.

Also, bumping up the version in tarball-internal.jenkinsfile is no longer required. This is done automatically.

Edited Mar 31, 2021 by Andrei Pavel

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information