doc: Replace deprecated keyserver
The keyserver <pool.sks-keyservers.net> has been deprecated. Replacing it with <keys.openpgp.org>.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
The keyserver <pool.sks-keyservers.net> has been deprecated. Replacing it with <keys.openpgp.org>.