chore(deps): update dependency webpack to v5 (#470)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [webpack](https://togithub.com/webpack/webpack) | devDependencies | major | [`^4.41.2` -> `^5.0.0`](https://renovatebot.com/diffs/npm/webpack/4.44.2/5.1.0) |

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### Release Notes

<details>
<summary>webpack/webpack</summary>

### [`v5.1.0`](https://togithub.com/webpack/webpack/releases/v5.1.0)

[Compare Source](https://togithub.com/webpack/webpack/compare/v5.0.0...v5.1.0)

### Features

-   expose `webpack` property from `Compiler`
-   expose `cleverMerge`, `EntryOptionPlugin`, `DynamicEntryPlugin`

### Bugfixes

-   missing `require("..").xxx` in try-catch produces a warning instead of an error now
-   handle reexports in concatenated modules correctly when they are side-effect-free
-   fix incorrect deprecation message for ModuleTemplate.hooks.hash

### [`v5.0.0`](https://togithub.com/webpack/webpack/releases/v5.0.0)

[Compare Source](https://togithub.com/webpack/webpack/compare/v4.44.2...v5.0.0)

[Announcement and changelog](https://webpack.js.org/blog/2020-10-10-webpack-5-release/)

</details>

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packages/google-cloud-tasks/package.json

@@ -64,7 +64,7 @@
     "sinon": "^9.0.1",
     "ts-loader": "^8.0.0",
     "typescript": "^3.8.3",
-    "webpack": "^4.41.2",
+    "webpack": "^5.0.0",
     "webpack-cli": "^4.0.0",
     "@microsoft/api-documenter": "^7.8.10",
     "@microsoft/api-extractor": "^7.8.10"