[design system] always set dimensions for Icon

[macfarlandian]

What needs to be done? Why does it need to be done?
The SVG icons will not be displayed properly in all supported browsers without an explicit size set. There are a few options for accomplishing this that can be evaluated:

1. set a default value
   • if this can't be the same for all icons, it may be more trouble than it's worth
2. require either size or height & width props
   • most explicit option but also a breaking change

In addition: height and width are currently applied as attributes rather than CSS properties. We should either transition those to CSS or ensure that the attribute values are not set in rem units (which is not supported as an SVG attribute) by detecting rems and converting them back to pixels. (polished has some utility functions that can help with this)

Additional context
See https://github.com/Recidiviz/recidiviz-data/pull/8259 as well as numerous React console warnings in Case Triage about unexpected rem units