Fix find_frontend_files script

This was broken
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/170174.

Node [requires][1] file extensions when used with the `import`
statement.

[1]: https://nodejs.org/docs/latest-v20.x/api/esm.html#esm_mandatory_file_extensions

scripts/frontend/find_frontend_files.mjs

@@ -5,7 +5,7 @@ import { fileURLToPath } from 'node:url';
 import Runtime from 'jest-runtime';
 import { readConfig } from 'jest-config';
 
-import createJestConfig from '../../jest.config.base';
+import createJestConfig from '../../jest.config.base.js';
 
 const ROOT = resolve(dirname(fileURLToPath(import.meta.url)), '../../');