move gnupg|gnupg2 from apt Depends to Recommends

apt doesn't need gnupg in its main execution paths to function,
especially the Release file verification is done with gpgv only.
It is only used by apt-key for advanced key management functionality
most user will never use nor need.

The intend is to demote it eventually to Suggests, but we opt here for a
staged downgrade as there are still third-party repositories out there
which require apt-key functionality without depending on gnupg (or apt
for that matter).