command/0.13upgrade: make confirmation more user-friendly (#25142)

I feel the current confirmation prompt for 0.13upgrade command is
ambiguous what is expected. Actually, when I used it for the first time,
I cancelled it by typing `y` instead of `yes`.

I believe it would be great if the 0.13upgrade command tell us the
expected value for confirmation like 0.12upgrade.

command/013_config_upgrade.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"context"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -14,6 +15,7 @@ import (
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/configs"
 	"github.com/hashicorp/terraform/internal/getproviders"
+	"github.com/hashicorp/terraform/terraform"
 	"github.com/hashicorp/terraform/tfdiags"
 	tfversion "github.com/hashicorp/terraform/version"
 	"github.com/zclconf/go-cty/cty"
@@ -148,7 +150,11 @@ command and dealing with them before running this command again.
 		if dir != "." {
 			query = fmt.Sprintf("Would you like to upgrade the module in %s?", dir)
 		}
-		v, err := c.Ui.Ask(query)
+		v, err := c.UIInput().Input(context.Background(), &terraform.InputOpts{
+			Id:          "approve",
+			Query:       query,
+			Description: `Only 'yes' will be accepted to confirm.`,
+		})
 		if err != nil {
 			diags = diags.Append(err)
 			c.showDiagnostics(diags)

command/013_config_upgrade_test.go

@@ -1,7 +1,6 @@
 package command
 
 import (
-	"bytes"
 	"io/ioutil"
 	"os"
 	"path"
@@ -215,10 +214,10 @@ func TestZeroThirteenUpgrade_confirm(t *testing.T) {
 	defer os.RemoveAll(td)
 	defer testChdir(t, td)()
 
+	// Ask input
+	defer testInteractiveInput(t, []string{"yes"})()
+
 	ui := new(cli.MockUi)
-	inputBuf := &bytes.Buffer{}
-	ui.InputReader = inputBuf
-	inputBuf.WriteString("yes")
 	c := &ZeroThirteenUpgradeCommand{
 		Meta: Meta{
 			testingOverrides: metaOverridesForProvider(testProvider()),
@@ -244,10 +243,10 @@ func TestZeroThirteenUpgrade_cancel(t *testing.T) {
 	defer os.RemoveAll(td)
 	defer testChdir(t, td)()
 
+	// Ask input
+	defer testInteractiveInput(t, []string{"no"})()
+
 	ui := new(cli.MockUi)
-	inputBuf := &bytes.Buffer{}
-	ui.InputReader = inputBuf
-	inputBuf.WriteString("no")
 	c := &ZeroThirteenUpgradeCommand{
 		Meta: Meta{
 			testingOverrides: metaOverridesForProvider(testProvider()),