Bump rack from 3.1.11 to 3.1.12 in /examples/v2/kitchensink (#403)

Bumps [rack](https://github.com/rack/rack) from 3.1.11 to 3.1.12.
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a
href="https://github.com/rack/rack/blob/main/CHANGELOG.md">rack's
changelog</a>.</em></p>
<blockquote>
<h2>[3.1.12] - 2025-03-11</h2>
<h3>Security</h3>
<ul>
<li><a
href="https://github.com/rack/rack/security/advisories/GHSA-7wqh-767x-r66v">CVE-2025-27610</a>
Local file inclusion in <code>Rack::Static</code>.</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/rack/rack/commit/e8f47608668d507e0f231a932fa37c9ca551c0a5"><code>e8f4760</code></a>
Bump patch version.</li>
<li><a
href="https://github.com/rack/rack/commit/413b8349f18032784b24b32e6c425eea14bb2a2e"><code>413b834</code></a>
Update changelog.</li>
<li><a
href="https://github.com/rack/rack/commit/70cb3228c755c2d066c108a9c59fbe67aff8e889"><code>70cb322</code></a>
Use a fully resolved file path when confirming if a file can be served
by `Ra...</li>
<li>See full diff in <a
href="https://github.com/rack/rack/compare/v3.1.11...v3.1.12">compare
view</a></li>
</ul>
</details>
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examples/v2/kitchensink/Gemfile.lock

@@ -15,7 +15,7 @@ GEM
     nio4r (2.7.4)
     puma (6.5.0)
       nio4r (~> 2.0)
-    rack (3.1.11)
+    rack (3.1.12)
     rack-protection (4.1.1)
       base64 (>= 0.1.0)
       logger (>= 1.6.0)