Enable form tracking for /users/sign_up/welcome

What does this MR do and why?

Solves https://gitlab.com/gitlab-org/gitlab/-/issues/359805

Screenshots or screen recordings

How to set up and validate locally

Ensure that you are "acting as saas"

export GITLAB_SIMULATE_SAAS=1

Navigate to /-/trial_registrations/new as non-signed user. Fill out the form and click Continue. User is redirected to /users/sign_up/welcome. Open Snowplow Debugger - click or type on fields.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.