Drop Debian 7/8/9; Add 10/11; Drop Ubuntu 14/16; Add 20.04

Signed-off-by: Christos Papageorgiou <christos.papageorgioy@gmail.com>

metadata.json

@@ -29,17 +29,15 @@
     {
       "operatingsystem": "Debian",
       "operatingsystemrelease": [
-        "7",
-        "8",
-        "9"
+        "10",
+        "11"
       ]
     },
     {
       "operatingsystem": "Ubuntu",
       "operatingsystemrelease": [
-        "14.04",
-        "16.04",
-        "18.04"
+        "18.04",
+        "20.04"
       ]
     },
     {