feat: update index file for mainnet v1.20.6

Author: MakerDAO-DevOps

api/index.json

@@ -1,6 +1,7 @@
 {
   "mainnet": {
     "all": [
+      "1.20.6",
       "1.20.5",
       "1.20.4",
       "1.20.3",
@@ -90,7 +91,7 @@
       "1.0.1",
       "1.0.0"
     ],
-    "active": "1.20.5"
+    "active": "1.20.6"
   },
   "goerli": {
     "all": [