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/eclipse-jetty.json

@@ -1,36 +1,5 @@
 {
-  "releases": {
-    "9.4": {
-      "name": "9.4"
-    },
-    "9.3": {
-      "name": "9.3"
-    },
-    "9.2": {
-      "name": "9.2"
-    },
-    "9.1": {
-      "name": "9.1"
-    },
-    "9.0": {
-      "name": "9.0"
-    },
-    "8": {
-      "name": "8"
-    },
-    "7": {
-      "name": "7"
-    },
-    "12": {
-      "name": "12"
-    },
-    "11": {
-      "name": "11"
-    },
-    "10": {
-      "name": "10"
-    }
-  },
+  "releases": {},
   "versions": {
     "12.0.21": {
       "name": "12.0.21",