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-cloud.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-cloud" width="16" height="16" aria-hidden="true"><path d="M2 7.25A5.225 5.225 0 0 1 7.25 2a5.222 5.222 0 0 1 4.767 3.029A4.472 4.472 0 0 1 16 9.5c0 2.505-1.995 4.5-4.5 4.5h-8A3.474 3.474 0 0 1 0 10.5c0-1.41.809-2.614 2.001-3.17Zm1.54.482a.75.75 0 0 1-.556.832c-.86.22-1.484.987-1.484 1.936 0 1.124.876 2 2 2h8c1.676 0 3-1.324 3-3s-1.324-3-3-3a.75.75 0 0 1-.709-.504A3.72 3.72 0 0 0 7.25 3.5C5.16 3.5 3.5 5.16 3.5 7.25c.002.146.014.292.035.436l.004.036.001.008Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-cloud" width="16" height="16" aria-hidden="true"><path d="M2 7.25A5.225 5.225 0 0 1 7.25 2a5.222 5.222 0 0 1 4.767 3.029A4.472 4.472 0 0 1 16 9.5c0 2.505-1.995 4.5-4.5 4.5h-8A3.474 3.474 0 0 1 0 10.5c0-1.41.809-2.614 2.001-3.17Zm1.54.482a.75.75 0 0 1-.556.832c-.86.22-1.484.987-1.484 1.936 0 1.124.876 2 2 2h8c1.676 0 3-1.324 3-3s-1.324-3-3-3a.75.75 0 0 1-.709-.504A3.72 3.72 0 0 0 7.25 3.5C5.16 3.5 3.5 5.16 3.5 7.25c.002.146.014.292.035.436l.004.036.001.008Z"/></svg>