fix(vercel): redirect sentry.dev root to sentry.io (#118076)

Visitors to `sentry.dev` (no subdomain) currently land on the Vercel
preview deployment of the app, which is confusing — it looks like the
real product but isn't.

This adds a host-scoped redirect rule so bare `sentry.dev` traffic is
sent to `sentry.io`. PR preview subdomains (e.g. `pr-1234.sentry.dev`)
are unaffected because the `has` host condition matches exactly on
`sentry.dev`.

Using a temporary (307) redirect since we may want to revisit this if
the domain gets repurposed.

---
[View Session in
Sentry](https://sentry.sentry.io/traces/?project=4510944073809921&query=gen_ai.conversation.id%3A%22slack%3AC8V02RHC7%3A1781819110.636419%22)

Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>

Author: ryan953

vercel.json

@@ -2,6 +2,15 @@
   "$schema": "https://openapi.vercel.sh/vercel.json",
   "version": 2,
 
+  "redirects": [
+    {
+      "source": "/(.*)",
+      "has": [{"type": "host", "value": "sentry.dev"}],
+      "destination": "https://sentry.io/$1",
+      "permanent": false
+    }
+  ],
+
   "rewrites": [
     {
       "source": "/(api|organization-avatar|team-avatar|avatar)/(.*)",