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/netbsd.json

@@ -1,8 +1,5 @@
 {
   "releases": {
-    "9": {
-      "name": "9"
-    },
     "8": {
       "name": "8",
       "eol": "2024-05-04"
@@ -27,9 +24,6 @@
       "name": "2",
       "eol": "2008-08-21"
     },
-    "10": {
-      "name": "10"
-    },
     "1": {
       "name": "1",
       "eol": "2006-05-17"