all: update supported versions

Go 1.9 and 1.10 produce binaries that don't run on the most recent version
of macOS. Drop these versions from the tested set.

Update minor revision where a newer one is available:
  1.15.9 -> 1.15.15
  1.16.1 -> 1.16.10

Add 1.17.3.

Bump go.mod version to go1.11.

Fixes golang/protobuf#1394.

Change-Id: I7850b86819d356891917d2987f080d3e7dbd7cc5
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/370055
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Florian Zenker <floriank@google.com>

go.mod

@@ -1,6 +1,6 @@
 module google.golang.org/protobuf
 
-go 1.9
+go 1.11
 
 require (
 	github.com/golang/protobuf v1.5.0

integration_test.go

@@ -38,7 +38,7 @@ var (
 	protobufVersion = "3.15.3"
 	protobufSHA256  = "" // ignored if protobufVersion is a git hash
 
-	golangVersions = []string{"1.9.7", "1.10.8", "1.11.13", "1.12.17", "1.13.15", "1.14.15", "1.15.9", "1.16.1"}
+	golangVersions = []string{"1.11.13", "1.12.17", "1.13.15", "1.14.15", "1.15.15", "1.16.10", "1.17.3"}
 	golangLatest   = golangVersions[len(golangVersions)-1]
 
 	staticcheckVersion = "2020.1.4"