chore: remove @nuxt/postcss8

[ineshbose]

closes #513

[what-the-diff]

PR Summary

• Elimination of the "@nuxt/postcss8" Dependency
  The team removed a piece of software called "@nuxt/postcss8" from the list of dependencies. This means that our product no longer relies on this specific software to function.

• Added Safety Measures in module.ts
  To avoid complications, the developers introduced additional error handling. This improvement will provide a smoother experience for users running our product on older versions of the software 'nuxt'. Specifically, it helps prevent problems that could occur when trying to install the "@nuxt/postcss8" module on nuxt versions that are lower than 2.16.

[Atinux]

We should update the documentation for Nuxt < 2.16 to also install @nuxt/postcss8 WDYT?

[nuxt-studio]

✅ Live Preview ready!

Name	Edit	Preview	Latest Commit
TailwindCSS	Edit on Studio ↗︎	View Live Preview	20c5326

[ineshbose]

Done 👍 Wasn't sure where we would specify these instructions for Nuxt 2.15, but I've added a callout that also links to https://github.com/nuxt/postcss8!