chore(deps): update dependency ts-loader to v7 (#390)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [ts-loader](https://github.com/TypeStrong/ts-loader) | devDependencies | major | [`^6.2.1` -> `^7.0.0`](https://renovatebot.com/diffs/npm/ts-loader/6.2.2/7.0.0) |

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### Release Notes

<details>
<summary>TypeStrong/ts-loader</summary>

### [`v7.0.0`](https://github.com/TypeStrong/ts-loader/blob/master/CHANGELOG.md#v700)

[Compare Source](https://github.com/TypeStrong/ts-loader/compare/v6.2.2...v7.0.0)

-   [Project reference support enhancements](https://github.com/TypeStrong/ts-loader/pull/1076) - thanks [@&#8203;sheetalkamat](https://github.com/sheetalkamat)!
-   Following the end of life of Node 8, `ts-loader` no longer supports Node 8 **BREAKING CHANGE**

</details>

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packages/google-cloud-texttospeech/package.json

@@ -59,7 +59,7 @@
     "null-loader": "^3.0.0",
     "pack-n-play": "^1.0.0-2",
     "sinon": "^9.0.1",
-    "ts-loader": "^6.2.1",
+    "ts-loader": "^7.0.0",
     "typescript": "^3.8.3",
     "webpack": "^4.41.2",
     "webpack-cli": "^3.3.10"