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/openzfs.json

@@ -1,21 +1,5 @@
 {
-  "releases": {
-    "2.3": {
-      "name": "2.3"
-    },
-    "2.2": {
-      "name": "2.2"
-    },
-    "2.1": {
-      "name": "2.1"
-    },
-    "2.0": {
-      "name": "2.0"
-    },
-    "0.8": {
-      "name": "0.8"
-    }
-  },
+  "releases": {},
   "versions": {
     "2.3.3": {
       "name": "2.3.3",