Use operatingsystemmajrelease instead of lsbmajdistrelease

manifests/params.pp

 class postfix::params {
   case $::osfamily {
     'RedHat': {
-      $seltype = $::lsbmajdistrelease ? {
+      $seltype = $::operatingsystemmajrelease ? {
         '4'   => 'etc_t',
         /5|6|7/ => 'postfix_etc_t',
         default => undef,