Update xeus-cpp to be compatible with xeus 6.0

[mcbarton]

With the recent release of xeus 6.0 the ci for xeus-cpp is failing on main due to the breaking changes made to xeus.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.06%. Comparing base (3002a8e) to head (c3f58d4).

Files with missing lines	Patch %	Lines
src/xinterpreter.cpp	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
- Coverage   81.25%   81.06%   -0.19%     
==========================================
  Files          21       21              
  Lines         864      866       +2     
  Branches       78       78              
==========================================
  Hits          702      702              
- Misses        162      164       +2     

Files with missing lines	Coverage Δ
src/xinterpreter.cpp	89.50% <50.00%> (-1.12%)	⬇️

Files with missing lines	Coverage Δ
src/xinterpreter.cpp	89.50% <50.00%> (-1.12%)	⬇️

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[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[anutosh491]

I would have just waited for @JohanMabille to make this change cause he's the most educated about the update to xeus 6 😬

But yeah let's ask him to review this right now as you've already made it !

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[JohanMabille]

The idea is also to always use helper functions to create the response messages instead of building them adhoc.

Also the kernelspec should be updated to contain the new field "kernel_protocol_version" and with value "5.6.0".

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"