Remove boxplot from supported diagram types

[mb]

I wanted to create a boxplot, but it doesn't seem possible. I think this should be reflected in the documentation (or are there plans to add support for boxplots again. I'd be even more happy with that ^^).

https://redash.io/help/user-guide/visualizations/visualization-types shows boxplot as being supported. Mark it as deprecated or remove it from the documentation to reflect current status.