Refs #37797 - ensure we can start PostgreSQL by forcing the old version

Something (f-m) might have already called switch-to postgresql:13, which
- upgraded packages
- tried to restart services

The later obviously fails as the on-disk data is in the wrong format.

hooks/boot/06-postgresql-upgrade-extensions.rb

@@ -2,18 +2,23 @@
 
 module PostgresqlUpgradeHookContextExtension
   def needs_postgresql_upgrade?(new_version)
-    File.read('/var/lib/pgsql/data/PG_VERSION').chomp.to_i < new_version.to_i
+    current_version.to_i < new_version.to_i
   rescue Errno::ENOENT
     false
   end
 
+  def current_version
+    File.read('/var/lib/pgsql/data/PG_VERSION').chomp
+  end
+
   def os_needs_postgresql_upgrade?
     el8? && needs_postgresql_upgrade?(13)
   end
 
   def postgresql_upgrade(new_version)
     logger.notice("Performing upgrade of PostgreSQL to #{new_version}")
 
+    execute!("dnf module switch-to postgresql:#{current_version} -y", false, true)
     start_services(['postgresql'])
     postgres_list = `runuser -l postgres -c 'psql --list --tuples-only --csv'`
     postgres_databases = CSV.parse(postgres_list)

spec/postgresql_upgrade_hook_context_extension_spec.rb

@@ -31,6 +31,7 @@
     subject { context.postgresql_upgrade(13) }
 
     before do
+      allow(File).to receive(:read).with('/var/lib/pgsql/data/PG_VERSION').and_return('12')
       allow(context).to receive(:logger).and_return(logger)
       allow(context).to receive(:'execute!')
       allow(context).to receive(:ensure_packages)