(SUP-2493) Remove support for SysVinit operating systems

This commit removes support for operating systems that do not use
SystemD. SystemD is a requirement of this module moving forward as the
cron jobs have been replaced with SystemD timers.

Author: Jarret Lavallee

metadata.json

@@ -14,39 +14,34 @@
     {
       "operatingsystem": "RedHat",
       "operatingsystemrelease": [
-        "6",
         "7",
         "8"
       ]
     },
     {
       "operatingsystem": "CentOS",
       "operatingsystemrelease": [
-        "6",
         "7",
         "8"
       ]
     },
     {
       "operatingsystem": "OracleLinux",
       "operatingsystemrelease": [
-        "6",
         "7",
         "8"
       ]
     },
     {
       "operatingsystem": "Scientific",
       "operatingsystemrelease": [
-        "6",
         "7",
         "8"
       ]
     },
     {
       "operatingsystem": "Ubuntu",
       "operatingsystemrelease": [
-        "16.04",
         "18.04"
       ]
     }