Adopt DFD3 for diagram components

[jgadsden]

Describe what problem your feature request solves: It would be good to adopt a more standard way of drawing the data flow diagram components

Describe the solution you'd like:

Adopt the v3 DFD described by @adamshostack One notable change would be rectangles with curved corners for processes instead of the existing circles

Additional context: This would be a good way of making the use of version 3.x more obvious, when version 3.x is released sometime in 2025 / 2026

[ericfitz]

+1