Increase PlantUML width/height limits
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### Proposal
Some customers require large diagrams when rendering PNG images with PlantUML. The current default width/height of 4096 is not sufficient. Diagrams will be cut off at the maximum height (or width). We should consider increasing the `PLANTUML_LIMIT_SIZE` limit on our PlantUML integration for GitLab.com.
See the [PlantUML FAQ](https://plantuml.com/faq#e689668a91b8d065). It's possible that this have memory implications, but we could start at something like 8192. This will likely need infrastructure to update this and ensure it scales.
{width=487 height=179}
{width=413 height=140}
{width=959 height=31}
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