chore(deps-dev): Bump postcss from 8.5.20 to 8.5.21 (#2027)

[//]: # (dependabot-start)
⚠️  **Dependabot is rebasing this PR** ⚠️ 

Rebasing might not happen immediately, so don't worry if this takes some
time.

Note: if you make any changes to this PR yourself, they will take
precedence over the rebase.

---

[//]: # (dependabot-end)

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.20 to
8.5.21.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/postcss/postcss/releases">postcss's
releases</a>.</em></p>
<blockquote>
<h2>8.5.21</h2>
<ul>
<li>Fixed childless at-rule losing semicolon before comment (by <a
href="https://github.com/sarathfrancis90"><code>@​sarathfrancis90</code></a>).</li>
<li>Fixed docs (by <a
href="https://github.com/isker"><code>@​isker</code></a>).</li>
</ul>
</blockquote>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a
href="https://github.com/postcss/postcss/blob/main/CHANGELOG.md">postcss's
changelog</a>.</em></p>
<blockquote>
<h2>8.5.21</h2>
<ul>
<li>Fixed childless at-rule losing semicolon before comment (by <a
href="https://github.com/sarathfrancis90"><code>@​sarathfrancis90</code></a>).</li>
<li>Fixed docs (by <a
href="https://github.com/isker"><code>@​isker</code></a>).</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/postcss/postcss/commit/28e0daf8f2fe5ba9e19ea3f8c27c8fe176f9419e"><code>28e0daf</code></a>
Release 8.5.21 version</li>
<li><a
href="https://github.com/postcss/postcss/commit/3d2b4e43e38274f233b5609d09687cadad8215d9"><code>3d2b4e4</code></a>
Update dependencies</li>
<li><a
href="https://github.com/postcss/postcss/commit/d197327e82e1a7d6dd9272effa3e4bfa91fdd20e"><code>d197327</code></a>
Fix childless at-rule losing its semicolon before a comment (<a
href="https://redirect.github.com/postcss/postcss/issues/2115">#2115</a>)</li>
<li><a
href="https://github.com/postcss/postcss/commit/0e360b749aa17a5a89baccf9c8e5db3c2978c21e"><code>0e360b7</code></a>
Fix mismatched JSDoc comments on Position (<a
href="https://redirect.github.com/postcss/postcss/issues/2114">#2114</a>)</li>
<li>See full diff in <a
href="https://github.com/postcss/postcss/compare/8.5.20...8.5.21">compare
view</a></li>
</ul>
</details>
<br />


[![Dependabot compatibility
score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=postcss&package-manager=npm_and_yarn&previous-version=8.5.20&new-version=8.5.21)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores)

Dependabot will resolve any conflicts with this PR as long as you don't
alter it yourself. You can also trigger a rebase manually by commenting
`@dependabot rebase`.

[//]: # (dependabot-automerge-start)
[//]: # (dependabot-automerge-end)

---

<details>
<summary>Dependabot commands and options</summary>
<br />

You can trigger Dependabot actions by commenting on this PR:
- `@dependabot rebase` will rebase this PR
- `@dependabot recreate` will recreate this PR, overwriting any edits
that have been made to it
- `@dependabot show <dependency name> ignore conditions` will show all
of the ignore conditions of the specified dependency
- `@dependabot ignore this major version` will close this PR and stop
Dependabot creating any more for this major version (unless you reopen
the PR or upgrade to it yourself)
- `@dependabot ignore this minor version` will close this PR and stop
Dependabot creating any more for this minor version (unless you reopen
the PR or upgrade to it yourself)
- `@dependabot ignore this dependency` will close this PR and stop
Dependabot creating any more for this dependency (unless you reopen the
PR or upgrade to it yourself)


</details>

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

package.json

@@ -134,7 +134,7 @@
     "istanbul": "^0.4.5",
     "istanbul-lib-coverage": "^3.2.2",
     "nyc": "^18.0.0",
-    "postcss": "^8.5.20",
+    "postcss": "^8.5.21",
     "react-hot-loader": "^4.13.1",
     "sass": "^1.101.3",
     "stylelint": "^17.14.1",