Bump mutex from 3.0.1 to 3.1.0 in /auto_submit (#3129)

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Bumps [mutex](https://github.com/hoylen/dart-mutex) from 3.0.1 to 3.1.0.
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/hoylen/dart-mutex/blob/master/CHANGELOG.md">mutex's changelog</a>.</em></p>
<blockquote>
<h2>3.1.0</h2>
<ul>
<li>Increased minimum Dart SDK to 2.15.0 for <code>unawaited</code> function.</li>
<li>Added development dependencies lints ^2.1.1 and pana: ^0.21.37.</li>
<li>Fixed code to remove lint warnings.</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/hoylen/dart-mutex/commit/84ca903a3ac863735e3228c75a212133621f680f"><code>84ca903</code></a> Added development dependencies lints ^2.1.1 and pana: ^0.21.37.</li>
<li><a href="https://github.com/hoylen/dart-mutex/commit/b9c797a34215d06307b1749beb919bee83e9ad5b"><code>b9c797a</code></a> Increased minimum Dart SDK to 2.15.0 for <code>unawaited</code> function.</li>
<li>See full diff in <a href="https://github.com/hoylen/dart-mutex/compare/v3.0.1...v3.1.0">compare view</a></li>
</ul>
</details>
<br />

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auto_submit/pubspec.yaml

@@ -28,7 +28,7 @@ dependencies:
   logging: 1.2.0
   retry: 3.1.2
   yaml: 3.1.2
-  mutex: 3.0.1
+  mutex: 3.1.0
 
 dev_dependencies:
   build_runner: 2.4.6