logstash: rename syslog facility to facility_name when it's a string

Such facility ids are generated by NetworkManager but the journalbeat index mapping is expecting a long (7.15.2)

Related to T3705

Test Plan

-logstash0:

diff origin/production/logstash0.internal.softwareheritage.org current/logstash0.internal.softwareheritage.org
*******************************************
  File[/etc/logstash/conf.d/filter.conf] =>
   parameters =>
     content =>
      @@ -31,4 +31,9 @@
           }
         } else {
      +    if [syslog][facility] !~ /^\d+$/ {
      +      mutate {
      +        rename => { "[syslog][facility]" => "[syslog][facility_name]" }
      +      }
      +    }
           if [@metadata][version] {
             mutate {
*******************************************
*** End octocatalog-diff on logstash0.internal.softwareheritage.org

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Migrated from D6665 (view on Phabricator)