Infrastructure: Bump 3rdparty/qtkeychain from 6e90eda to 53a8e6a (Mudlet#9126)

Bumps
[3rdparty/qtkeychain](https://github.com/frankosterfeld/qtkeychain) from
`6e90eda` to `53a8e6a`.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/frankosterfeld/qtkeychain/commit/53a8e6a31f7d24bc58d91515cc7fcbb34af554db"><code>53a8e6a</code></a>
CMake: link with Qt6 cmake targets</li>
<li><a
href="https://github.com/frankosterfeld/qtkeychain/commit/2b5a3f408f1d6887bb87ce0c24428f7bf30b637a"><code>2b5a3f4</code></a>
cmake: default to c++17</li>
<li><a
href="https://github.com/frankosterfeld/qtkeychain/commit/2598fe6ebc649d65dcd79d457b5bddb26fe0d5ad"><code>2598fe6</code></a>
Add legacy support for KWallet maps</li>
<li>See full diff in <a
href="https://github.com/frankosterfeld/qtkeychain/compare/6e90eda69bd8c5e2f7dd64cd9a5d2392382b8525...53a8e6a31f7d24bc58d91515cc7fcbb34af554db">compare
view</a></li>
</ul>
</details>
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