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/graalvm-ce.json

@@ -1,24 +1,5 @@
 {
-  "releases": {
-    "24": {
-      "name": "24"
-    },
-    "23": {
-      "name": "23"
-    },
-    "22": {
-      "name": "22"
-    },
-    "21": {
-      "name": "21"
-    },
-    "20": {
-      "name": "20"
-    },
-    "17": {
-      "name": "17"
-    }
-  },
+  "releases": {},
   "versions": {
     "24.0.1": {
       "name": "24.0.1",