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/gitea-gitlab.svg

@@ -1 +1 @@
-<svg viewBox="0 0 48 48" class="svg gitea-gitlab" width="16" height="16" aria-hidden="true"><path fill="#e53935" d="m24 43-8-23h16z"/><path fill="#ff7043" d="m24 43 18-23H32z"/><path fill="#e53935" d="m37 5 5 15H32z"/><path fill="#ffa726" d="m24 43 18-23 3 8z"/><path fill="#ff7043" d="M24 43 6 20h10z"/><path fill="#e53935" d="M11 5 6 20h10z"/><path fill="#ffa726" d="M24 43 6 20l-3 8z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 48 48" class="svg gitea-gitlab" width="16" height="16" aria-hidden="true"><path fill="#e53935" d="m24 43-8-23h16z"/><path fill="#ff7043" d="m24 43 18-23H32z"/><path fill="#e53935" d="m37 5 5 15H32z"/><path fill="#ffa726" d="m24 43 18-23 3 8z"/><path fill="#ff7043" d="M24 43 6 20h10z"/><path fill="#e53935" d="M11 5 6 20h10z"/><path fill="#ffa726" d="M24 43 6 20l-3 8z"/></svg>