Use released packages until nightly release packages are fixed

The release packages on nightlies.puppet.com contain the wrong GPG key and since
those repos won't be updated, just use the previously released packages for now.

Author: joshcooper

acceptance/tests/test_upgrade_puppet6_to_puppet7.rb

@@ -37,7 +37,8 @@ class { puppet_agent:
   end
 
   agents_only.each do |agent|
-    set_up_initial_agent_on(agent, puppet_collection: 'puppet6-nightly') do
+    # REMIND: PA-7431 use nightly repos once those release packages are fixed
+    set_up_initial_agent_on(agent, puppet_collection: 'puppet6') do
       step '(Agent) Change agent environment to testing environment' do
         on(agent, puppet("config --section agent set environment #{puppet_testing_environment}"))
         on(agent, puppet('config --section user set environment production'))

acceptance/tests/test_upgrade_puppet7_to_puppet8.rb

@@ -37,7 +37,8 @@ class { puppet_agent:
   end
 
   agents_only.each do |agent|
-    set_up_initial_agent_on(agent, puppet_collection: 'puppet7-nightly') do
+    # REMIND: PA-7431 use nightly repos once those release packages are fixed
+    set_up_initial_agent_on(agent, puppet_collection: 'puppet7') do
       step '(Agent) Change agent environment to testing environment' do
         on(agent, puppet("config --section agent set environment #{puppet_testing_environment}"))
         on(agent, puppet('config --section user set environment production'))