Bump google/mirror-branch-action from 1.0 to 2.0 (#126600)

Bumps [google/mirror-branch-action](https://github.com/google/mirror-branch-action) from 1.0 to 2.0.
<details>
<summary>Commits</summary>
<ul>
<li><a href="30c52ee21f"><code>30c52ee</code></a> Update dependencies.</li>
<li><a href="4544d8a090"><code>4544d8a</code></a> Update deprecated action environment</li>
<li><a href="884625cd18"><code>884625c</code></a> Bump node-fetch from 2.6.0 to 2.6.1</li>
<li><a href="d9fb13d47a"><code>d9fb13d</code></a> Bump <code>@​actions/core</code> from 1.2.4 to 1.2.6</li>
<li><a href="4083f5d474"><code>4083f5d</code></a> Unify placeholders in README.md</li>
<li><a href="394f8d4604"><code>394f8d4</code></a> Bump lodash from 4.17.15 to 4.17.19 (<a href="https://redirect.github.com/google/mirror-branch-action/issues/3">#3</a>)</li>
<li><a href="5d7016e8e9"><code>5d7016e</code></a> The sample <code>uses:</code> line should reference <a href="https://github.com/v1"><code>@​v1</code></a>.0 (<a href="https://redirect.github.com/google/mirror-branch-action/issues/1">#1</a>)</li>
<li>See full diff in <a href="c6b07e441a...30c52ee21f">compare view</a></li>
</ul>
</details>
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.github/workflows/mirror.yml

@@ -21,7 +21,7 @@ jobs:
     steps:
       - name: Mirror action step
         id: mirror
-        uses: google/mirror-branch-action@c6b07e441a7ffc5ae15860c1d0a8107a3a151db8
+        uses: google/mirror-branch-action@30c52ee21f5d3bd7fb28b95501c11aae7f17eebb
         with:
           github-token: ${{ secrets.FLUTTERMIRRORINGBOT_TOKEN }}
           source: 'master'