Changes $modal-md and $modal-lg to 500px and 800px, down from 600px and 900px. This was mainly done to ensure proper sizing between small and medium breakpoints for the default modal sizing. After making the change for that, 900px seemed far too high for the larger modal option, so I changed that to feel a little more appropriate.
Fixes #19973 (closed), nullifying #18443 and #21110.