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/mxlinux.json

@@ -1,30 +1,5 @@
 {
-  "releases": {
-    "23": {
-      "name": "23"
-    },
-    "21": {
-      "name": "21"
-    },
-    "19": {
-      "name": "19"
-    },
-    "18": {
-      "name": "18"
-    },
-    "17": {
-      "name": "17"
-    },
-    "16": {
-      "name": "16"
-    },
-    "15": {
-      "name": "15"
-    },
-    "14": {
-      "name": "14"
-    }
-  },
+  "releases": {},
   "versions": {
     "23.6": {
       "name": "23.6",