fix(deps): update dependency google/cloud-dialogflow to ^0.21 (GoogleCloudPlatform#1407)

[![WhiteSource Renovate](https://app.renovatebot.com/images/banner.svg)](https://renovatebot.com)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [google/cloud-dialogflow](https://togithub.com/googleapis/google-cloud-php-dialogflow) | require | minor | `^0.20` -> `^0.21` |

---

### Release Notes


googleapis/google-cloud-php-dialogflow

### [`v0.21.0`](https://togithub.com/googleapis/google-cloud-php-dialogflow/releases/v0.21.0)

[Compare Source](https://togithub.com/googleapis/google-cloud-php-dialogflow/compare/v0.20.1...v0.21.0)

#### google/cloud-dialogflow 0.21.0

##### Features

-   added Automated agent reply type and allow cancellation flag for partial response feature. ([#​4109](https://www.github.com/googleapis/google-cloud-php/issues/4109)) ([889bb4c](https://www.github.com/googleapis/google-cloud-php/commit/889bb4c2c66d46137231db149ee7f1cb4fe78474))



---

### Configuration

📅 **Schedule**: At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] If you want to rebase/retry this PR, check this box.

---

This PR has been generated by [WhiteSource Renovate](https://renovate.whitesourcesoftware.com). View repository job log [here](https://app.renovatebot.com/dashboard#github/GoogleCloudPlatform/php-docs-samples).

Author: renovate-bot

dialogflow/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "google/cloud-dialogflow": "^0.20",
+        "google/cloud-dialogflow": "^0.21",
         "symfony/console": "^3.1"
     },
     "autoload": {