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/ghc.json

@@ -1,45 +1,5 @@
 {
-  "releases": {
-    "9.8": {
-      "name": "9.8"
-    },
-    "9.6": {
-      "name": "9.6"
-    },
-    "9.4": {
-      "name": "9.4"
-    },
-    "9.2": {
-      "name": "9.2"
-    },
-    "9.12": {
-      "name": "9.12"
-    },
-    "9.10": {
-      "name": "9.10"
-    },
-    "9.0": {
-      "name": "9.0"
-    },
-    "8.8": {
-      "name": "8.8"
-    },
-    "8.6": {
-      "name": "8.6"
-    },
-    "8.4": {
-      "name": "8.4"
-    },
-    "8.2": {
-      "name": "8.2"
-    },
-    "8.10": {
-      "name": "8.10"
-    },
-    "8.0": {
-      "name": "8.0"
-    }
-  },
+  "releases": {},
   "versions": {
     "9.10.2": {
       "name": "9.10.2",