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 <[email protected]>
Commit-Queue: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>

go.mod

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