Fix encoding issues with plantuml blocks

What does this MR do and why?

Fix issue with PlantUML diagrams that mixed up unicode characters.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In a GitLab.com project, click wiki on the left bar menu then Create a new page

2. In the text section copy the code block :

    ```plantuml
    A -> B : Text with norwegian characters: æøå
    ``` 

3. Click on Rich text, the conversion of æøå should not be wrong.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #375344 (closed)