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/apache-groovy.json

@@ -1,18 +1,5 @@
 {
-  "releases": {
-    "4.0": {
-      "name": "4.0"
-    },
-    "3.0": {
-      "name": "3.0"
-    },
-    "2.5": {
-      "name": "2.5"
-    },
-    "2.4": {
-      "name": "2.4"
-    }
-  },
+  "releases": {},
   "versions": {
     "3.0.25": {
       "name": "3.0.25",