…prints

fabb72b contrib: Remove xpired 522739F6 key (MarcoFalke)
faeab66 contrib: Replace developer keys with list of pgp fingerprints (MarcoFalke)

Pull request description:

  Having to host a copy of the keys in this repo was a common source of discussion and distraction, caused by problems such as:

  * Outdated keys. Unclear whether and when to replace by fresh copies.
  * Unclear when to add a key of a new developer or Gitian builder.

  The problems are solved by
  * Having no keys but only the fingerprints
  * Adding a rule of thumb, when to add a new key

  <strike>Moving the keys to a different repo solves none of these issues, but since the keys are not bound to releases or git branches of Bitcoin Core, they should live somewhere else.

  Obviously, all keys are hosted and distributed on key servers, but were added to the repo solely for convenience and redundancy.

  Moving the mirror of those keys to a different repo makes it less distracting to update them -- let's say -- prior to every major release.

  I updated our `doc/release-process.md` to reflect the new location.

  DEPENDS_ON bitcoin-core/gitian.sigs#621
  </strike>

Tree-SHA512: c00795a07603190e26dc4526f6ce11e492fb048dc7ef54b38f859b77dcde25f58ec4449f5cf3f85a5e9c2dd2743bde53f7ff03c8eccf0d75d51784a6b164e47d