Use the new "Deprecated:" syntax for all instances of HasPrefix.
This is a follow-up to http://golang.org/cl/28413 which only modified path_unix.go.

In this CL, we avoid mentioning that strings.HasPrefix should be used since
that function is still subtly wrong in security applications.

See http://golang.org/cl/5712045 for more information.

Fixes #18355

Change-Id: I0d0306152cd0b0ea5110774c2c78117515b9f5cd
Reviewed-on: https://go-review.googlesource.com/34554
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>