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/istio.json

@@ -1,14 +1,5 @@
 {
   "releases": {
-    "1.9": {
-      "name": "1.9"
-    },
-    "1.8": {
-      "name": "1.8"
-    },
-    "1.7": {
-      "name": "1.7"
-    },
     "1.26": {
       "name": "1.26",
       "eol": "2026-01-31",
@@ -77,15 +68,6 @@
       "name": "1.13",
       "eol": "2022-10-12",
       "supportedKubernetesVersions": "1.20 - 1.23"
-    },
-    "1.12": {
-      "name": "1.12"
-    },
-    "1.11": {
-      "name": "1.11"
-    },
-    "1.10": {
-      "name": "1.10"
     }
   },
   "versions": {