Migrate edit_prediction_provider setting before updating its value to 'zed' during onboarding (#24781)

This fixes a bug where we'd update your settings to an invalid state if
you were using the old `inline_completion_provider` setting, then
onboarded to Zeta, then migrated your settings.

Release Notes:

- N/A

Co-authored-by: Michael Sloan <mgsloan@gmail.com>
Co-authored-by: Agus Zubiaga <hi@aguz.me>