Remove invalid releases automatically (#466)

Automatically remove empty releases or releases which are released in the future, regardless of the product.

Also refactored a bit releasedata.py to use better names and clarify types.

releases/zabbix.json

@@ -1,33 +1,5 @@
 {
-  "releases": {
-    "7.4": {
-      "name": "7.4"
-    },
-    "7.2": {
-      "name": "7.2"
-    },
-    "7.0": {
-      "name": "7.0"
-    },
-    "6.4": {
-      "name": "6.4"
-    },
-    "6.2": {
-      "name": "6.2"
-    },
-    "6.0": {
-      "name": "6.0"
-    },
-    "5.4": {
-      "name": "5.4"
-    },
-    "5.0": {
-      "name": "5.0"
-    },
-    "4.0": {
-      "name": "4.0"
-    }
-  },
+  "releases": {},
   "versions": {
     "7.4.0": {
       "name": "7.4.0",