Keep (add if not existing) xmlns attribute for generated SVG images (#23410)

Fix #23409 

Developers could browse & preview the local SVG images files directly.

It still has clear output.

![image](https://user-images.githubusercontent.com/2114189/224317107-f4b26c76-e36a-4e80-9eee-d8dc2e16421f.png)


![image](https://user-images.githubusercontent.com/2114189/224317527-2d4ca131-978c-4933-b071-4bae483f06e1.png)

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Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <leon@kske.dev>

public/img/svg/octicon-webhook.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-webhook" width="16" height="16" aria-hidden="true"><path d="M5.5 4.25a2.25 2.25 0 0 1 4.5 0 .75.75 0 0 0 1.5 0 3.75 3.75 0 1 0-6.14 2.889l-2.272 4.258a.75.75 0 0 0 1.324.706L7 7.25a.75.75 0 0 0-.309-1.015A2.25 2.25 0 0 1 5.5 4.25Z"/><path d="M7.364 3.607a.75.75 0 0 1 1.03.257l2.608 4.349a3.75 3.75 0 1 1-.628 6.785.75.75 0 0 1 .752-1.299 2.25 2.25 0 1 0-.033-3.88.75.75 0 0 1-1.03-.256L7.107 4.636a.75.75 0 0 1 .257-1.03Z"/><path d="M2.9 8.776A.75.75 0 0 1 2.625 9.8 2.25 2.25 0 1 0 6 11.75a.75.75 0 0 1 .75-.751h5.5a.75.75 0 0 1 0 1.5H7.425a3.751 3.751 0 1 1-5.55-3.998.75.75 0 0 1 1.024.274Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-webhook" width="16" height="16" aria-hidden="true"><path d="M5.5 4.25a2.25 2.25 0 0 1 4.5 0 .75.75 0 0 0 1.5 0 3.75 3.75 0 1 0-6.14 2.889l-2.272 4.258a.75.75 0 0 0 1.324.706L7 7.25a.75.75 0 0 0-.309-1.015A2.25 2.25 0 0 1 5.5 4.25Z"/><path d="M7.364 3.607a.75.75 0 0 1 1.03.257l2.608 4.349a3.75 3.75 0 1 1-.628 6.785.75.75 0 0 1 .752-1.299 2.25 2.25 0 1 0-.033-3.88.75.75 0 0 1-1.03-.256L7.107 4.636a.75.75 0 0 1 .257-1.03Z"/><path d="M2.9 8.776A.75.75 0 0 1 2.625 9.8 2.25 2.25 0 1 0 6 11.75a.75.75 0 0 1 .75-.751h5.5a.75.75 0 0 1 0 1.5H7.425a3.751 3.751 0 1 1-5.55-3.998.75.75 0 0 1 1.024.274Z"/></svg>