Upgrade go-github v61 -> v71 (#34385)

There will be a possible bug when migrating from Github
https://github.com/google/go-github/issues/3229
This PR upgrades go-github from v61 to v71 to resolve that problem.

contrib/backport/backport.go

@@ -18,7 +18,7 @@ import (
 	"strings"
 	"syscall"
 
-	"github.com/google/go-github/v61/github"
+	"github.com/google/go-github/v71/github"
 	"github.com/urfave/cli/v2"
 	"gopkg.in/yaml.v3"
 )