Fix wiki diagram encoding

What does this MR do and why?

Fix wiki diagram encoding

related issue:

• #412451 (closed)
• #412984 (closed)
• https://jihulab.com/gitlab-cn/gitlab/-/issues/3333

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. run gdk local
2. Visit any project wiki create pages such as http://127.0.0.1:3000/flightjs/Flight/-/wikis/home?view=create
3. Click the diagram button.

4. Insert some unicode text

5. save and quit diagram editor
6. save wiki page
7. expect to display diagram with unicode text correctly

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.