Bump postcss from 8.5.5 to 8.5.6 (#13798)

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.5 to 8.5.6.
<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>8.5.6</h2>
<ul>
<li>Fixed <code>ContainerWithChildren</code> type discriminating (by <a
href="https://github.com/Goodwine"><code>@​Goodwine</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>8.5.6</h2>
<ul>
<li>Fixed <code>ContainerWithChildren</code> type discriminating (by <a
href="https://github.com/Goodwine"><code>@​Goodwine</code></a>).</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/postcss/postcss/commit/91d6eb5c3d1ca8acb4e8e3926005acf2b066c211"><code>91d6eb5</code></a>
Release 8.5.6 version</li>
<li><a
href="https://github.com/postcss/postcss/commit/65ffc55117bf4289b1f977986ed76fad402641b1"><code>65ffc55</code></a>
Update dependencies</li>
<li><a
href="https://github.com/postcss/postcss/commit/ecd20eb7f9587d63e3f3348b768aec0e9fb000d3"><code>ecd20eb</code></a>
Fix ContainerWithChildren to allow discriminating the node type by
comparing ...</li>
<li>See full diff in <a
href="https://github.com/postcss/postcss/compare/8.5.5...8.5.6">compare
view</a></li>
</ul>
</details>
<br />


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

Author: dependabot[bot]