maintainer-pgp-guide: remove keyserver instructions

maintainer-pgp-guide: remove keyserver instructions

Keyservers are largely a thing of the past with the replacement systems
like keys.openpgp.net specifically designed to offer no support for the
web of trust. Remove all sections that talk about keyservers and add a
small section with the link to kernel.org documentation that talks about
using the kernel.org public key repository.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220727-docs-pgp-guide-v2-2-e3e6954affb6@linuxfoundation.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>