fix: change return urls in quickstarts

[jacwil]

Change the default return url for Quickstarts to welcome page instead of site root.

Related issue(s)

Issue: Sign In redirects to root page of the self service ui.

More context: The Kratos SelfService UI used by the Quickstarts does not serve a root page but instead returns a json error: {"error":{"code":404,"status":"Not Found","message":"Requested url does not match any rules"}}.

Steps to reproduce:

1. Start Kratos and Oathkeeper. Example: docker compose -f quickstart.yml -f quickstart-oathkeeper.yml up --build --force-recreate
2. Go to the welcome page and then Sign Up.
3. Upon completing account creation you are redirected to site root and presented with the json "Not Found" error.
4. Go to the welcome page after creating an account and Sign Out.
5. Go to the welcome page and Sign In. Upon successful sign in you are redirected to site root and presented with the json "Not Found" error.

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• I have added tests that prove my fix is effective or that my feature
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