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/opnsense.json

@@ -1,69 +1,5 @@
 {
-  "releases": {
-    "25.1": {
-      "name": "25.1"
-    },
-    "24.7": {
-      "name": "24.7"
-    },
-    "24.1": {
-      "name": "24.1"
-    },
-    "23.7": {
-      "name": "23.7"
-    },
-    "23.1": {
-      "name": "23.1"
-    },
-    "22.7": {
-      "name": "22.7"
-    },
-    "22.1": {
-      "name": "22.1"
-    },
-    "21.7": {
-      "name": "21.7"
-    },
-    "21.1": {
-      "name": "21.1"
-    },
-    "20.7": {
-      "name": "20.7"
-    },
-    "20.1": {
-      "name": "20.1"
-    },
-    "19.7": {
-      "name": "19.7"
-    },
-    "19.1": {
-      "name": "19.1"
-    },
-    "18.7": {
-      "name": "18.7"
-    },
-    "18.1": {
-      "name": "18.1"
-    },
-    "17.7": {
-      "name": "17.7"
-    },
-    "17.1": {
-      "name": "17.1"
-    },
-    "16.7": {
-      "name": "16.7"
-    },
-    "16.1": {
-      "name": "16.1"
-    },
-    "15.7": {
-      "name": "15.7"
-    },
-    "15.1": {
-      "name": "15.1"
-    }
-  },
+  "releases": {},
   "versions": {
     "25.1.10": {
       "name": "25.1.10",