all: use io.Seek* instead of deprecated os.SEEK_*

all: use io.Seek* instead of deprecated os.SEEK_*

These are available since Go 1.7. The version used for bootstrap is Go
1.17 for Go 1.20.

For #44505

Change-Id: I497c9f617baefdeb273cd115b08b6e31bd10aad2
Reviewed-on: https://go-review.googlesource.com/c/go/+/421634
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>