Documentation/process/maintainer-pgp-guide: Replace broken link to PGP path finder

Documentation/process/maintainer-pgp-guide: Replace broken link to PGP path finder

PGP pathfinder[1], which is suggested for finding a trust path to
unknown PGP keys by 'maintainer-pgp-guide.rst', is not working now.
This commit replaces it with other available tools.

[1] https://pgp.cs.uu.nl/

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210812095030.4704-2-sj38.park@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>