Add redirect to fix upgrade guide URLs (#988)

Author: rmainwork

content/vault/v1.20.x/redirects.jsonc

@@ -547,5 +547,11 @@
     "source": "/vault/docs/:version(v1\\.*\\.x)/v:ver(1\\.*\\.x)/:slug*",
     "destination": "/vault/docs/:version/:slug*",
     "permanent": true
+  },
+  // Fix v prefix in upgrade guide URLs (e.g., upgrade-to-v1.16.x -> upgrade-to-1.16.x)
+  {
+    "source": "/vault/docs/:version(v1\\.(?:9|1[0-8])\\.x)/upgrading/upgrade-to-v:ver(1\\.(?:9|1[0-8])\\.x)",
+    "destination": "/vault/docs/:version/upgrading/upgrade-to-:ver",
+    "permanent": true
   }
-]
+]