Change external domain used in offboarding test to be more reliable

[knolleary]

Fixes #5019

The offboarding test that verifies the user gets redirected to the configured URL is a bit flaky as it redirects to Google which sometimes returns something unexpected.

This changes the test to redirect to nodered.org and far more predictable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.40%. Comparing base (e1e344c) to head (84b9f65).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5023   +/-   ##
=======================================
  Coverage   78.40%   78.40%           
=======================================
  Files         329      329           
  Lines       15478    15478           
  Branches     3574     3574           
=======================================
  Hits        12136    12136           
  Misses       3342     3342           

Flag	Coverage Δ
backend	78.40% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.