Bump futures-util from 0.3.19 to 0.3.21 (#326)

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Bumps [futures-util](https://github.com/rust-lang/futures-rs) from 0.3.19 to 0.3.21.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a href="https://github.com/rust-lang/futures-rs/releases">futures-util's releases</a>.</em></p>
<blockquote>
<h2>0.3.21</h2>
<ul>
<li>Fix potential data race in <code>FlattenUnordered</code> that introduced in 0.3.20 (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2566">#2566</a>)</li>
</ul>
<h2>0.3.20</h2>
<ul>
<li>Fix stacked borrows violations when <code>-Zmiri-tag-raw-pointers</code> is enabled. This raises MSRV of <code>futures-task</code> to 1.45. (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2548">#2548</a>, <a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2550">#2550</a>)</li>
<li>Change <code>FuturesUnordered</code> to respect yielding from future (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2551">#2551</a>)</li>
<li>Add <code>StreamExt::{flatten_unordered, flat_map_unordered}</code> (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2083">#2083</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md">futures-util's changelog</a>.</em></p>
<blockquote>
<h1>0.3.21 - 2022-02-06</h1>
<ul>
<li>Fix potential data race in <code>FlattenUnordered</code> that introduced in 0.3.20 (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2566">#2566</a>)</li>
</ul>
<h1>0.3.20 - 2022-02-06</h1>
<p>NOTE: This release has been yanked due to a bug fixed in 0.3.21.</p>
<ul>
<li>Fix stacked borrows violations when <code>-Zmiri-tag-raw-pointers</code> is enabled. This raises MSRV of <code>futures-task</code> to 1.45. (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2548">#2548</a>, <a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2550">#2550</a>)</li>
<li>Change <code>FuturesUnordered</code> to respect yielding from future (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2551">#2551</a>)</li>
<li>Add <code>StreamExt::{flatten_unordered, flat_map_unordered}</code> (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2083">#2083</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/rust-lang/futures-rs/commit/fc1e3250219170e31cddb8857a276cba7dd08d44"><code>fc1e325</code></a> Release 0.3.21</li>
<li><a href="https://github.com/rust-lang/futures-rs/commit/20279ebbfcd326e83161e44a2cf5afe1bfc3b074"><code>20279eb</code></a> <code>FlattenUnordered</code>: improve wakers behavior (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2566">#2566</a>)</li>
<li><a href="https://github.com/rust-lang/futures-rs/commit/75dca5ae7fabe6b7073558a8fc6793ee5caa7057"><code>75dca5a</code></a> Fix MSRV in futures-task readme</li>
<li><a href="https://github.com/rust-lang/futures-rs/commit/55281c8c8de6308ba97ce8b4ffd754ba204409a0"><code>55281c8</code></a> Release 0.3.20</li>
<li><a href="https://github.com/rust-lang/futures-rs/commit/591b982af835fc5e0ffb5104bbb9e942b2ed0631"><code>591b982</code></a> Redefine executor and compat modules in futures crate (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2564">#2564</a>)</li>
<li><a href="https://github.com/rust-lang/futures-rs/commit/94b508bfcb1d1f986f34f9b12d799e5a73f65f22"><code>94b508b</code></a> Basic <code>StreamExt::{flatten_unordered, flat_map_unordered}</code> impls (<a href="https://github-redirect.dependabot.com/rust-lang/futures-rs/issues/2083">#2083</a>)</li>
<li><a href="https://github.com/rust-lang/futures-rs/commit/dca16fafa597d8e0075a7b693904c6f2ce0322a4"><code>dca16fa</code></a> Do not auto-create PR on fork</li>
<li><a href="https://github.com/rust-lang/futures-rs/commit/a9795a9243b0e6b836f71cbe9661bbb35bb17a7f"><code>a9795a9</code></a> Automatically creates PR when no_atomic_cas.rs needs to be updated</li>
<li><a href="https://github.com/rust-lang/futures-rs/commit/4841888d68eb8a7796ccd9c9f7a9dc677586e054"><code>4841888</code></a> Update comments in build scripts</li>
<li><a href="https://github.com/rust-lang/futures-rs/commit/85706b6d67aa30e1f8380f8101c14cf43be7500e"><code>85706b6</code></a> Clean up ci/no_atomic_cas.sh</li>
<li>Additional commits viewable in <a href="https://github.com/rust-lang/futures-rs/compare/0.3.19...0.3.21">compare view</a></li>
</ul>
</details>
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