feat!: upgrade to @openedx/paragon v23 and design tokens

[MaxFrank13]

Changes

• Upgrades peer dependency on @openedx/paragon to ^23.0.0 (BREAKING CHANGE)
• Changes @edx/brand npm alias to refer to @edx/elm-theme in dev dependencies.
• Example app index.scss file has been updated to pull from the new location of the CSS files for openedx/paragon and edx/elm-theme

How to test locally

• Pull down this branch
• Make sure you are logged in on your local instance
• Run npm ci and npm start
• Go to localhost:8080
• To confirm that design tokens are being used, you can inspect the CSS for any component and look for CSS variables
• Another giveaway that elm-theme is being used correctly would be to look at the button component for "Sign Up" -- it should have completely rounded edges as shown in the screenshot

Screenshot

New header:

Old header: