Use keyrings for apt sources on Debian/Ubuntu

[xolyu]

Use nginx's official key https://nginx.org/keys/nginx_signing.key instead of hardcoded key ID with deprecated apt-key method.

nginx's maintainer will rotate PGP keys every two years:

Going forward, we will rotate the PGP signing key every two years.
(source)

So, it would be better to use key URL with keyring infrastructure instead of a regularly expiring key that is hardcoded with it's ID in this repository and is not updated quickly.

That would also solve #1624

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https://github.com/voxpupuli/puppet-nginx/blob/master/manifests/package/debian.pp#L34

[xolyu]

There is already a pull request, but not merged, yet.

#1610