Bump submodules/googletest from 6dae7eb to 71815bb (#76)

Bumps [submodules/googletest](https://github.com/google/googletest) from
`6dae7eb` to `71815bb`.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/google/googletest/commit/71815bbf7de6e10c11821d654a2fae2cf42de0f7"><code>71815bb</code></a>
Automated Code Change</li>
<li><a
href="https://github.com/google/googletest/commit/a1e255a582377e1006bb88a408ac3f933ba7c916"><code>a1e255a</code></a>
Optional(): Add support for std::optional&lt;&gt;-like types lacking
bool conversion.</li>
<li>See full diff in <a
href="https://github.com/google/googletest/compare/6dae7eb4a5c3a169f3e298392bff4680224aa94a...71815bbf7de6e10c11821d654a2fae2cf42de0f7">compare
view</a></li>
</ul>
</details>
<br />


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