Remove muli

[petemill]

Changes

Removes all occurances of the Muli font and replaces with Poppins. The biggest change is in the theme file, which other projects use, but really mainly (only?) brave-core.

Test plan

Make sure storybook runs with no error. I verified this. Main visual (manual) tests will be in brave-core.

Link / storybook path to visual changes

• https://brave-ui-petemill.vercel.app/

Integration

• [ ] Does this contain changes to src/components or src/

  • [ ] Will you publish to npm immediately after this PR, or wait until sometime in the future?
  • [ ] Incompatible API change to something existing (major version increase)
  • [ ] Adding new backwards-compatible functionality? (minor version increase)
  • [ ] Fixing a bug backwards-compatibly? (patch version increase)

• [ ] Does this contain changes to src/features for brave-core?

  • [ ] Are there non backwards-compatible changes required for brave-core? Do not merge until brave-core PR is approvable. Link to brave-core PR:
  • [ ] Will you create brave-core PR to update to this commit after it is merged?
  • [ ] Wants uplift to brave-core feature branch?
    • When uplift-approved, merge to brave-core-0.VV.x feature branch
    • Create additional brave-core PRs for each feature branch to update commit

[boocmp]

I don't actually understand what is StoryBook and how I can use it. The last time I showed an build from my PR to @aguscruiz, he recommended to change the font weight for Poppins. I'm not sure, but it looks like we need to change the weights in the storybook.

[petemill]

@boocmp I think we'll need to build brave-core with this brave-ui ref in order to see what changes would be neccessary, and I think most or all of those changes would be in brave-core code. I'll push to your branch in https://github.com/brave/brave-core/pull/12933 an update to the brave-ui ref so that we can get a browser build and Storybook