When I deployed server-side changes, I put the upgrade handler at /ts2021
instead of /switch. We could move the server to /switch, but ts2021 seems
more specific and better, but I don't feel strongly.
Updates #3488
Change-Id: Ifbf8ea60a815fd2fa1bfbe1b7af1ac2a27218354
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>