all: set required version to 1.17

1.17 is the oldest version we are testing and thus this is the oldest
version we can guarantee this repository works for. We also plan to
use features of newer Go version (up to 1.17) in following CLs.

Other changed to the go.mod file were introduced by `go mod tidy`

Change-Id: I79034d0a5f9c9ac9608308d01bc7e35725b2b592
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/547337
Commit-Queue: Michael Stapelberg <stapelberg@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

go.mod

@@ -1,8 +1,10 @@
 module google.golang.org/protobuf
 
-go 1.11
+go 1.17
 
 require (
 	github.com/golang/protobuf v1.5.0
 	github.com/google/go-cmp v0.5.5
 )
+
+require golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 // indirect