Replace x/net/context with context of stdlib

server/api.go

 package server
 
 import (
+	"context"
 	"errors"
 	"fmt"
 
 	"golang.org/x/crypto/bcrypt"
 
-	// go-grpc doesn't use the standard library's context.
-	// https://github.com/grpc/grpc-go/issues/711
-	"golang.org/x/net/context"
-
 	"github.com/dexidp/dex/api"
 	"github.com/dexidp/dex/pkg/log"
 	"github.com/dexidp/dex/server/internal"