Fix PlantUML integration in GFM
What does this MR do?
Use "lang" attribute instead of "class" in PlantumlFilter.
Does this MR meet the acceptance criteria?
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Changelog entry added, if necessary [ ] Documentation created/updated[ ] API support added- Tests
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Added for this feature/bug -
All builds are passing
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Conform by the merge request performance guides -
Conform by the style guides -
Branch has no merge conflicts with master
(if it does - rebase it please) -
Squashed related commits together
What are the relevant issue numbers?
Fixes #30557 (closed)