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-rocket.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-feed-rocket" width="16" height="16" aria-hidden="true"><path d="M8 16A8 8 0 1 1 8 0a8 8 0 0 1 0 16Zm3.031-12a4.38 4.38 0 0 0-3.097 1.283l-.23.229c-.156.157-.308.32-.452.49H5.65a.876.876 0 0 0-.746.417l-.856 1.388a.377.377 0 0 0 .21.556l1.552.477 1.35 1.35.478 1.553a.374.374 0 0 0 .555.21l1.389-.855a.876.876 0 0 0 .416-.746V8.747c.17-.144.333-.295.49-.452l.23-.23A4.379 4.379 0 0 0 12 4.969v-.093A.876.876 0 0 0 11.124 4Zm-5.107 7.144h-.001a.809.809 0 0 0-1.33-.881c-.395.394-.564 1.258-.62 1.62a.12.12 0 0 0 .035.108.12.12 0 0 0 .108.035c.362-.056 1.226-.225 1.62-.619a.803.803 0 0 0 .188-.263Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-feed-rocket" width="16" height="16" aria-hidden="true"><path d="M8 16A8 8 0 1 1 8 0a8 8 0 0 1 0 16Zm3.031-12a4.38 4.38 0 0 0-3.097 1.283l-.23.229c-.156.157-.308.32-.452.49H5.65a.876.876 0 0 0-.746.417l-.856 1.388a.377.377 0 0 0 .21.556l1.552.477 1.35 1.35.478 1.553a.374.374 0 0 0 .555.21l1.389-.855a.876.876 0 0 0 .416-.746V8.747c.17-.144.333-.295.49-.452l.23-.23A4.379 4.379 0 0 0 12 4.969v-.093A.876.876 0 0 0 11.124 4Zm-5.107 7.144h-.001a.809.809 0 0 0-1.33-.881c-.395.394-.564 1.258-.62 1.62a.12.12 0 0 0 .035.108.12.12 0 0 0 .108.035c.362-.056 1.226-.225 1.62-.619a.803.803 0 0 0 .188-.263Z"/></svg>