Documentation: update maintainer-pgp-guide for latest best practices

Freshen up the maintainer PGP guide:

- Bump minimum GnuPG version requirement from 2.2 to 2.4, since 2.2 is
  no longer maintained
- All major hardware tokens now support Curve25519, so remove outdated
  ECC support callouts
- Update hardware device recommendations (Nitrokey Pro 2 -> Nitrokey 3)
- Broaden backup media terminology (USB thumb drive -> external media)
- Update wording to follow vale's linter recommendations
- Various minor wording improvements for clarity

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Reviewed-by: Paul Barker <paul@pbarker.dev>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20250902-pgp-guide-updates-v1-1-62ac7312d3f9@linuxfoundation.org>