docs: update nodejs middleware instructions in README for x402-next

[dngpng]

Description

• Updated the README for x402-next with regard to the workaround when accessing Mainnet.
  • Since Next.js has made the nodejs middleware support stable since v15.5, there is no need to install a canary version and enable an experimental configuration in next.config.js
• Use GitHub-flavored Markdown's alert syntax for improved rendering of "important notes".

Tests

n/a

Checklist

• [x] I have formatted and linted my code
• [x] All new and existing tests pass
• [x] My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

[cb-heimdall]

🟡 Heimdall Review Status

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