Point /register link at the actual deployment hostname

Match the corrected backend host (actions-caching-registration.vercel.app) in the
not-registered notice, and refresh the vendored gradle-actions-caching to 0.9.2.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

sources/vendor/gradle-actions-caching/package.json

@@ -1,6 +1,6 @@
 {
   "name": "gradle-actions-caching",
-  "version": "0.9.1",
+  "version": "0.9.2",
   "type": "module",
   "main": "./index.js",
   "types": "./index.d.ts",