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/openwrt.json

@@ -1,27 +1,5 @@
 {
-  "releases": {
-    "24.10": {
-      "name": "24.10"
-    },
-    "23.05": {
-      "name": "23.05"
-    },
-    "22.03": {
-      "name": "22.03"
-    },
-    "21.02": {
-      "name": "21.02"
-    },
-    "19.07": {
-      "name": "19.07"
-    },
-    "18.06": {
-      "name": "18.06"
-    },
-    "17.01": {
-      "name": "17.01"
-    }
-  },
+  "releases": {},
   "versions": {
     "24.10.2": {
       "name": "24.10.2",