docs: warn about syncVercelEnvVars conflict in vercel integration page

Author: isshaddad

docs/vercel-integration.mdx

@@ -104,6 +104,12 @@ The following variables are excluded from the Vercel → Trigger.dev sync:
 
 You can control sync behavior per-variable from your project's Vercel settings. Deselecting a variable prevents its value from being updated during future syncs.
 
+<Warning>
+  If you are experiencing incorrectly populated environment variables, check that you are not using
+  the `syncVercelEnvVars` build extension in your `trigger.config.ts`. This extension is deprecated
+  and conflicts with the Vercel integration's built-in env var syncing. Remove it if present.
+</Warning>
+
 ### Supabase and Neon database branching
 
 If you use [Supabase Branching](https://supabase.com/docs/guides/deployment/branching) or [Neon Database Branching](https://neon.tech/docs/guides/branching-intro) for preview environments, disable syncing for database env vars on the Environment Variables page and use the [syncSupabaseEnvVars](/config/extensions/syncEnvVars#syncsupabaseenvvars) or [syncNeonEnvVars](/config/extensions/syncEnvVars#syncneonenvvars) build extensions instead. These extensions automatically resolve the correct branch-specific credentials at build time.