Add apt to dependencies in metadata.json

Prior to this commit, apt was required in the fixutures but not in
metadata.json. Version 7.2.0 was chosen as the lower bounds of the apt
module version because that is when Debian 10 support was added.

Fixes voxpupuli#1374

metadata.json

@@ -8,6 +8,10 @@
   "project_page": "http://github.com/voxpupuli/puppet-nginx",
   "issues_url": "https://github.com/voxpupuli/puppet-nginx/issues",
   "dependencies": [
+    {
+      "name": "puppetlabs/apt",
+      "version_requirement": ">= 7.2.0 < 8.0.0"
+    },
     {
       "name": "puppetlabs/concat",
       "version_requirement": ">= 4.1.0 < 7.0.0"