Bump syn from 2.0.53 to 2.0.55 (#2349)

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.53 to 2.0.55.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/dtolnay/syn/releases">syn's
releases</a>.</em></p>
<blockquote>
<h2>2.0.55</h2>
<ul>
<li>Restore compatibility with rustc 1.56 through 1.59 (<a
href="https://redirect.github.com/dtolnay/syn/issues/1603">#1603</a>)</li>
</ul>
<h2>2.0.54</h2>
<ul>
<li>Fix dead code warning in generated code when using
<code>custom_keyword!</code> with syn's &quot;printing&quot; feature
disabled (<a
href="https://redirect.github.com/dtolnay/syn/issues/1602">#1602</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/dtolnay/syn/commit/d4c4811bd58fd22b4cc68988166aee2132ffb3be"><code>d4c4811</code></a>
Release 2.0.55</li>
<li><a
href="https://github.com/dtolnay/syn/commit/21d9e5f6639c762a75bfa9013131f629e9099494"><code>21d9e5f</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1603">#1603</a>
from dtolnay/compat</li>
<li><a
href="https://github.com/dtolnay/syn/commit/6e8a372d605af4af48d4b3dcf2f41792777f2b67"><code>6e8a372</code></a>
Restore compatibility with rustc 1.56 through 1.59</li>
<li><a
href="https://github.com/dtolnay/syn/commit/a5d02b1afe7641dfaadd660be32bb9822b74f47b"><code>a5d02b1</code></a>
Release 2.0.54</li>
<li><a
href="https://github.com/dtolnay/syn/commit/c218a352e8a8aa39643a0d76de0136546b9a6265"><code>c218a35</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1602">#1602</a>
from dtolnay/deadcode</li>
<li><a
href="https://github.com/dtolnay/syn/commit/2ddfdc666678db6187b44beeb73d67c21484ab37"><code>2ddfdc6</code></a>
Suppress dead code lint on custom token's span field</li>
<li><a
href="https://github.com/dtolnay/syn/commit/e4f3216b6c12593ee6d3e3fa57eb794d86e4b793"><code>e4f3216</code></a>
Update test suite to nightly-2024-03-23</li>
<li><a
href="https://github.com/dtolnay/syn/commit/fa563a42e26fb72ebd48114bdc9332554d1cdf3f"><code>fa563a4</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1601">#1601</a>
from dtolnay/vistest</li>
<li><a
href="https://github.com/dtolnay/syn/commit/cfb9bb460c6f036f6241e8dcf283b9c8a7683f5e"><code>cfb9bb4</code></a>
Add test of unnamed tuple struct field with inherited visibility</li>
<li><a
href="https://github.com/dtolnay/syn/commit/73c308793774fe9b8cf60fa625a4c00c6e11ef17"><code>73c3087</code></a>
Update test suite to nightly-2024-03-22</li>
<li>Additional commits viewable in <a
href="https://github.com/dtolnay/syn/compare/2.0.53...2.0.55">compare
view</a></li>
</ul>
</details>
<br />


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