fix: update role of NcHeaderMenu

[kristian-zendato]

☑️ Resolves

• Fix: https://github.com/nextcloud-gmbh/customer-feature-requests/issues/1591
• Related PR: fix: make top nav bar right elements within a nav nextcloud/server#63239

Summary

Update role of NcHeaderMenu to menuitem

🚧 Tasks

• ...

🏁 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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.98%. Comparing base (0e0af10) to head (7c65224).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8874      +/-   ##
==========================================
- Coverage   52.23%   51.98%   -0.25%     
==========================================
  Files         115      120       +5     
  Lines        4009     4338     +329     
  Branches     1181     1289     +108     
==========================================
+ Hits         2094     2255     +161     
- Misses       1635     1766     +131     
- Partials      280      317      +37     

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[ShGKme]

1. With role="menuitem" here, isNav is still not valid. If we deprecated isNav prop as @susnux proposed, all the isNav handling must be removed. Otherwise we have an invalid state, where menuitem are items of the navigation instead if the menu.
2. This PR is compatible with the linked server PR for Nextcloud 35. However, it breaks all the previous versions semantics, where the header is not a manubar. We need to either add a new prop here, or check the server version, assuming, this is server version defined.

See also: nextcloud/server#63239 (comment)