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/spring-framework.json

@@ -23,21 +23,6 @@
       "releaseDate": "2020-10-27",
       "eol": "2023-06-30",
       "eoes": "2029-06-30"
-    },
-    "5.2": {
-      "name": "5.2"
-    },
-    "5.1": {
-      "name": "5.1"
-    },
-    "5.0": {
-      "name": "5.0"
-    },
-    "4.3": {
-      "name": "4.3"
-    },
-    "3.2": {
-      "name": "3.2"
     }
   },
   "versions": {