[dPWA] Rename web app manifest launch_handler route_to field as client_mode

This CL renames route_to to client_mode along with its associated
values according to spec change:
https://github.com/WICG/sw-launch/pull/60

This is a mechanical rename:
route_to: auto => client_mode: auto
route_to: new-client => client_mode: navigate-new
route_to: existing-client-navigate => client_mode: navigate-existing
route_to: existing-client-retain => client_mode: focus-existing

This change removes backwards compatibility with route_to and the
previously supported navigate_existing_client syntax.

Bug: 1349650
Change-Id: Ia2d4729647eeaff97884818ac1af8c240bf4dd6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3806920
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1031713}