example/aws/proxyWithTLSClientCert: Use Transport.ForceAttemptHTTP2…

… to enable HTTP/2 (#4014)

This can remove `golang.org/x/net/http2` dependency.

This field was added at Go 1.13. This example file now required
Go 1.15+. So we can use this field safely.
Ref: https://golang.org/doc/go1.13#net/http

example/aws/proxyWithTLSClientCert/main.go

@@ -17,7 +17,6 @@ import (
  "github.com/aws/aws-sdk-go/aws"
  "github.com/aws/aws-sdk-go/aws/session"
  "github.com/aws/aws-sdk-go/service/s3"
- "golang.org/x/net/http2"
 )
 
 // Example of creating an HTTP Client configured with a client TLS
@@ -52,10 +51,6 @@ func main() {
  tr := defaultHTTPTransport()
  tr.TLSClientConfig = tlsCfg
 
- // re-enable HTTP/2 because modifing TLS config will prevent auto support
- // for HTTP/2.
- http2.ConfigureTransport(tr)
-
  // Configure the SDK's session with the HTTP client with TLS client
  // certificate support enabled. This session will be used to create all
  // SDK's API clients.
@@ -120,5 +115,6 @@ func defaultHTTPTransport() *http.Transport {
  IdleConnTimeout: 90 * time.Second,
  TLSHandshakeTimeout: 10 * time.Second,
  ExpectContinueTimeout: 1 * time.Second,
+ ForceAttemptHTTP2: true,
  }
 }