Update on close mint extension lesson

[cpudvido]

Problem

The existing lesson on the Close Mint Extension was outdated, with content and examples no longer reflecting the latest developments in the Solana ecosystem. Additionally, the lesson needed improvements to its structure and clarity to ensure better understanding and usability for learners.

Summary of Changes

I updated the lesson to include the latest information on the Token extension program. The lesson now features revised content that aligns with the current standards and best practices in the Solana developer ecosystem. I also improved the clarity and organization of the material to provide a more engaging and informative learning experience. This includes updated code examples, explanations, and references to new tools and features.
external pull request:
starter code pull request
solution code pull request

Fixes #

[mikemaccana]

Closing this - there are some small fixes here but it mostly seems like changes just to change things, sometimes making things worse.