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/silverstripe.json

@@ -40,40 +40,10 @@
       "eoas": "2024-10-31",
       "eol": "2025-04-30"
     },
-    "5.1": {
-      "name": "5.1"
-    },
-    "5.0": {
-      "name": "5.0"
-    },
     "4.13": {
       "name": "4.13",
       "eoas": "2024-04-17",
       "eol": "2025-06-10"
-    },
-    "4.12": {
-      "name": "4.12"
-    },
-    "4.11": {
-      "name": "4.11"
-    },
-    "4.10": {
-      "name": "4.10"
-    },
-    "3.7": {
-      "name": "3.7"
-    },
-    "3.1": {
-      "name": "3.1"
-    },
-    "3.0": {
-      "name": "3.0"
-    },
-    "2.4": {
-      "name": "2.4"
-    },
-    "2.3": {
-      "name": "2.3"
     }
   },
   "versions": {