increase Mermaid MAX_CHAR_LIMIT to a more reasonable default

What does this MR do?

The currently implemented MAX_CHAR_LIMIT for Mermaid is set to just 2000 which is way too low for a lot of use-cases especially when e.g. making nodes clickable in which case all link URLs will also count towards the total character count of the diagram. This problem was initially addressed in #27173 (closed) (based on a default set by the Mermaid project) and there's currently an open issue for it at #207229.

By now, the Mermaid project has raised its default maxTextSize drastically from 5000 to 50000 characters per diagram.
➡️ https://github.com/mermaid-js/mermaid/blob/master/src/defaultConfig.js#L57

This MR ensures that GitLab reflects the changes made in Mermaid to also raise its MAX_CHAR_LIMIT to 50000 which will allow diagrams to be shown again without the need of clicking buttons for lazy loading.

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

The affected test was updated.