Renovate configuration: Enable go mod tidy (#4246)

Renovate configuration: Add a package rule making all go updates run `go
mod tidy` after the dependency bump.

This happens as a post update step. That is, the dependency is bumped in
the local branch created by Renovate, then `go mod tidy` is invoked thus
guaranteeing synchronization between `go.mod` and `go.sum` files.

Author: pfcoperez

renovate.json

@@ -14,6 +14,10 @@
         "*"
       ]
     },
+    {
+      "matchManagers": ["gomod"],
+      "postUpdateOptions": ["gomodTidy"]
+    },
     {
       "matchPackageNames": ["next", "react", "react-*", "@types/react", "@types/react-*", "eslint", "eslint-config-next"],
       "matchManagers": ["npm"],