[Callouts] Match latest Primer styles and colors

[seshrs]

Context

The current Primer Spec callout colors were inspired by GitHub's dark mode colors (and were included in the original implementation #111). However, shortly after I merged the PR, GitHub updated their dark-mode callout styles!

Initially, I wasn't a fan of the new colors — they felt a bit too "flat". However, the styles have since grown on me, and I think it's time to match GitHub's styles.

This PR updates all callout colors to match the colors from GitHub's Primer Styles for their Alert component.

I also changed the demo callouts slightly to exclude italic styles.

Validation

	Before	After
Light mode
Dark mode

I think the new styles are more legible, especially in dark mode.

[github-actions[bot]]

The spec from this PR is available at https://preview.sesh.rs/previews/eecs485staff/primer-spec/211/.

^{(Available until Fri Oct 21 2022.)}

[awdeorio]

I think the new styles are more legible, especially in dark mode.

I agree!