fix ubuntu 18.04 invalid repo

Postgresql no longer specifically supports Ubuntu 18.04, but still
supports Debian 10 which can be used until Puppet drops support for Ubuntu
18.04.

manifests/repo/apt_postgresql_org.pp

@@ -10,13 +10,20 @@
 
   $_baseurl = pick($postgresql::repo::baseurl, $default_baseurl)
 
+  # Puppet still supports Ubuntu 18.04, but Postgresql no longer provides
+  # specific builds for it. Use the equivalent Debian release.
+  $_release = $facts['os']['distro']['codename'] ? {
+    'bionic' => 'buster',
+    /(.+)/   => $1,
+  }
+
   apt::pin { 'apt_postgresql_org':
     originator => 'apt.postgresql.org',
     priority   => 500,
   }
   -> apt::source { 'apt.postgresql.org':
     location     => $_baseurl,
-    release      => "${facts['os']['distro']['codename']}-pgdg",
+    release      => $_release,
     repos        => 'main',
     architecture => $facts['os']['architecture'],
     key          => {

spec/classes/repo_spec.rb

@@ -3,11 +3,25 @@
 require 'spec_helper'
 
 describe 'postgresql::repo' do
-  include_examples 'Debian 11'
+  context 'on debian 11' do
+    include_examples 'Debian 11'
 
-  describe 'with no parameters' do
-    it 'instantiates apt_postgresql_org class' do
-      expect(subject).to contain_class('postgresql::repo::apt_postgresql_org')
+    describe 'with no parameters' do
+      it 'instantiates apt_postgresql_org class' do
+        expect(subject).to contain_class('postgresql::repo::apt_postgresql_org')
+      end
+
+      it { is_expected.to contain_apt__source('apt.postgresql.org').with_release(facts[:os]['distro']['codename']) }
+
+      it { is_expected.to contain_apt__pin('apt_postgresql_org') }
+    end
+  end
+
+  context 'on ubuntu 18.04' do
+    include_examples 'Ubuntu 18.04'
+
+    describe 'with no parameters' do
+      it { is_expected.to contain_apt__source('apt.postgresql.org').with_release('buster') }
     end
   end
 end