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-boot.json

@@ -30,38 +30,11 @@
       "eol": "2024-06-30",
       "eoes": "2025-06-30"
     },
-    "3.0": {
-      "name": "3.0"
-    },
     "2.7": {
       "name": "2.7",
       "releaseDate": "2022-05-19",
       "eol": "2023-06-30",
       "eoes": "2029-06-30"
-    },
-    "2.6": {
-      "name": "2.6"
-    },
-    "2.5": {
-      "name": "2.5"
-    },
-    "2.4": {
-      "name": "2.4"
-    },
-    "2.3": {
-      "name": "2.3"
-    },
-    "2.2": {
-      "name": "2.2"
-    },
-    "2.1": {
-      "name": "2.1"
-    },
-    "2.0": {
-      "name": "2.0"
-    },
-    "1.5": {
-      "name": "1.5"
     }
   },
   "versions": {