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/perl.json

@@ -1,33 +1,5 @@
 {
-  "releases": {
-    "5.42": {
-      "name": "5.42"
-    },
-    "5.40": {
-      "name": "5.40"
-    },
-    "5.38": {
-      "name": "5.38"
-    },
-    "5.36": {
-      "name": "5.36"
-    },
-    "5.34": {
-      "name": "5.34"
-    },
-    "5.32": {
-      "name": "5.32"
-    },
-    "5.30": {
-      "name": "5.30"
-    },
-    "5.28": {
-      "name": "5.28"
-    },
-    "5.26": {
-      "name": "5.26"
-    }
-  },
+  "releases": {},
   "versions": {
     "5.42.0": {
       "name": "5.42.0",