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-feed-discussion.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-feed-discussion" width="16" height="16" aria-hidden="true"><path d="M8 16A8 8 0 1 1 8 0a8 8 0 0 1 0 16ZM4 5v5a1 1 0 0 0 1 1h1v1.5a.5.5 0 0 0 .854.354L8.707 11H11a1 1 0 0 0 1-1V5a1 1 0 0 0-1-1H5a1 1 0 0 0-1 1Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-feed-discussion" width="16" height="16" aria-hidden="true"><path d="M8 16A8 8 0 1 1 8 0a8 8 0 0 1 0 16ZM4 5v5a1 1 0 0 0 1 1h1v1.5a.5.5 0 0 0 .854.354L8.707 11H11a1 1 0 0 0 1-1V5a1 1 0 0 0-1-1H5a1 1 0 0 0-1 1Z"/></svg>