Bump google/mirror-branch-action from 1.0 to 2.0 (#1811)

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="https://github.com/google/mirror-branch-action/commit/30c52ee21f5d3bd7fb28b95501c11aae7f17eebb"><code>30c52ee</code></a>
Update dependencies.</li>
<li><a
href="https://github.com/google/mirror-branch-action/commit/4544d8a0903fb21cfbcf4642b793e47f90783d5a"><code>4544d8a</code></a>
Update deprecated action environment</li>
<li><a
href="https://github.com/google/mirror-branch-action/commit/884625cd18fadb1bdb646df951a7b486a5d9f3fc"><code>884625c</code></a>
Bump node-fetch from 2.6.0 to 2.6.1</li>
<li><a
href="https://github.com/google/mirror-branch-action/commit/d9fb13d47a8b349b51d369518c798306a6b42669"><code>d9fb13d</code></a>
Bump <code>@​actions/core</code> from 1.2.4 to 1.2.6</li>
<li><a
href="https://github.com/google/mirror-branch-action/commit/4083f5d474abe94ca3b49400e1c55d1d825f08dd"><code>4083f5d</code></a>
Unify placeholders in README.md</li>
<li><a
href="https://github.com/google/mirror-branch-action/commit/394f8d4604fee868b2e256f4ac8ea148a4292806"><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="https://github.com/google/mirror-branch-action/commit/5d7016e8e9b3ef486f0a0f6dfbbb18c175c0c699"><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="https://github.com/google/mirror-branch-action/compare/c6b07e441a7ffc5ae15860c1d0a8107a3a151db8...30c52ee21f5d3bd7fb28b95501c11aae7f17eebb">compare
view</a></li>
</ul>
</details>
<br />


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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: 'main'