Update the maintainer PGP guide

This series updates the guide to match terminology used in the upstream
"protecting code integrity" guide and brings the documentation in line
with the latest developments in the GnuPG world:

- uses "Certify key" instead of "master key" terms to remove common
  confusion that the "Certify key" is somehow able to restore lost
  private subkeys
- removes keyserver instructions because keyservers have largely gone
  semi-extinct due to GDPR enforcement and just general neglect
- adds a link to the kernel.org PGP keyring documentation
- updates information about ECC curve support among the devices the
  guide talks about (Yubikeys are able to use ED25519 curves with the
  latest firmware updates)
- adds a section on using PGP-signed patches with b4 and patatt

Link: https://github.com/lfit/itpol/blob/master/protecting-code-integrity.md
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>

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Changes in v2:
- Rebase on v5.19.
- Small wording changes based on feedback.
- Link to v1: https://lore.kernel.org/r/20220727-docs-pgp-guide-v1-0-c48fb06cb9af@linuxfoundation.org