A11y: Hardcoded h5 creates accessability issues

[alexdryden]

Titles for the the diagrams are hardcoded as h5, which causes improper nesting hierarchies (see WCAG 2.2 Success Criterion 1.3.1, Technique G141).
(tagging: reported by @wolfsbane868 )

[jimsafley]

The heading level for the diagram title is currently hardcoded as h5, which I agree isn't ideal. However, the correct heading level depends on where the block is placed in the page structure, which varies by site and can't be determined by the module itself. Fixing this properly would require a configurable heading level setting, which adds complexity we're not ready to take on at this time. I'm leaving this open for future consideration.

[alexdryden]

I agree that dynamically detecting the appropriate heading level is not practical. Does the diagram title need to be a heading at all? Wouldn't the most appropriate solution be to enclose the diagram in a figure and use figcaption for the title and description?