chore(css): fix typo in postcss-config browserslist and update list

[phloxic]

• s/last 3 major version/last 3 major versions/
• sync with current browserslistrc, babelrc, rollup_config

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[welcome]

💖 Thanks for opening this pull request! 💖

Things that will help get your PR across the finish line:

• Run npm run lint -- --errors locally to catch formatting errors earlier.
• Include tests when adding/changing behavior.
• Include screenshots and animated GIFs whenever possible.

We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[adrums86]

Merging to get this into the next release. Thanks for this PR!

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉