chore(docs): correct min supported Nextcloud version

[ShGKme]

☑️ Resolves

• Correct docs/index.md to be up to date with README.md, missing changes from:
  • docs: v9 will only provide visual compatibility with Nextcloud 32+ #7287
  • chore: revert breaking changes and keep Nextcloud 31 support #7353
• Alternative: use README.md as index.md

🖼️ Screenshots

🏚️ Before	🏡 After

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[ShGKme]

/backport to stable8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.57%. Comparing base (e945848) to head (bf39007).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7774   +/-   ##
=======================================
  Coverage   51.57%   51.57%           
=======================================
  Files          96       96           
  Lines        3141     3141           
  Branches      862      863    +1     
=======================================
  Hits         1620     1620           
  Misses       1273     1273           
  Partials      248      248           

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