build(deps): bump postcss from 8.5.14 to 8.5.15 (#974)

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.14 to
8.5.15.
<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>8.5.15</h2>
<ul>
<li>Fixed declaration parsing performance (by <a
href="https://github.com/homanp"><code>@​homanp</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>8.5.15</h2>
<ul>
<li>Fixed declaration parsing performance (by <a
href="https://github.com/homanp"><code>@​homanp</code></a>).</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/postcss/postcss/commit/eae46db765d752cf8f40c4fa2b0b85030079c43d"><code>eae46db</code></a>
Release 8.5.15 version</li>
<li><a
href="https://github.com/postcss/postcss/commit/79508ffa59e42c02056aca61b88bc393c8b516c4"><code>79508ff</code></a>
Update CI actions</li>
<li><a
href="https://github.com/postcss/postcss/commit/b128e2131288a411c6e28071d0929542c49e74eb"><code>b128e21</code></a>
Speed up declaration parsing by avoiding creating new array on each
token</li>
<li><a
href="https://github.com/postcss/postcss/commit/9825dca02c33cf610e2a842be767468b67fbecf9"><code>9825dca</code></a>
Fix code format</li>
<li><a
href="https://github.com/postcss/postcss/commit/55789c865281e2be194fa5b4e41dd046be3a2307"><code>55789c8</code></a>
Update dependencies</li>
<li><a
href="https://github.com/postcss/postcss/commit/84fbbe9009cb3cc3bbb4cc3a9b65d468f4844d95"><code>84fbbe9</code></a>
Install older pnpm action for old Node.js</li>
<li><a
href="https://github.com/postcss/postcss/commit/9f860bd78ec1dbc4f0ae72d693f03f956baa38cb"><code>9f860bd</code></a>
Revert pnpm action for old Node.js</li>
<li><a
href="https://github.com/postcss/postcss/commit/08771986d47359545f502e009763e223b66bfcf6"><code>0877198</code></a>
Update CI actions</li>
<li><a
href="https://github.com/postcss/postcss/commit/b2d1a335cea818f8b27e5cfb90147648afe3e582"><code>b2d1a33</code></a>
Fix linter warnings</li>
<li><a
href="https://github.com/postcss/postcss/commit/0700dac92283bc259977dff2743ca74a00f58267"><code>0700dac</code></a>
Merge pull request <a
href="https://redirect.github.com/postcss/postcss/issues/2088">#2088</a>
from rootvector2/add-oss-fuzz-harness</li>
<li>Additional commits viewable in <a
href="https://github.com/postcss/postcss/compare/8.5.14...8.5.15">compare
view</a></li>
</ul>
</details>
<br />


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