Change "First Mile" to "Registration Flow"

Why is this change being made?

The term "First Mile" was initially intended to be used to avoid confusion between various Growth efforts focused on the user onboarding. It appears as though it has created more confusion, however, as we often have to qualify it with (the registration flow) or similar and when referencing it with folks outside of Growth it is not a well understood term.

I am proposing that we simplify down to calling it the "Registration Flow".

If we approve this change, we'll also want to update the label "first mile" that exists in the gitlab-org project.

Author Checklist

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