Update offical instructions on how to get the PGP key

The official method outlined on the "Release Channels and Signing Keys" page which explains how to get the PGP key via the admin@f-droid.org.jar is apparently depreciated.

Apparently that JAR is not being updated with the new key: this 6 moth old bug is closed but the JAR key remains 2 years out of date with no indication that the JAR will be updated. I can only take it that the JAR isn't being updated anymore?

The new method listed on that bug is to use Ubuntu key server.

If that is now the preferred official method to get the key could we update the Release Channels and Signing Keys page to reflect those changes? Thank you