go.mod: upgrade to go 1.17

This change was produced using 'go mod tidy -go=1.17'
with a go command built at CL 315210.

This activates lazy loading, and updates the go.mod file to maintain
the lazy-loading invariants (namely, including an explicit requirement
for every package transitively imported by the main module).

Note that this does *not* prevent users with earlier go versions from
successfully building packages from this module.

For golang/go#36460.

Change-Id: Idb3d8d056d41bb254d0b5d9b2699ac33a51081f3
Reviewed-on: https://go-review.googlesource.com/c/sync/+/316130
Auto-Submit: Bryan Mills <bcmills@google.com>
Commit-Queue: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

go.mod

@@ -1 +1,3 @@
 module golang.org/x/sync
+
+go 1.17