Pin nlohmann_json to 3.12.0

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

The latest release of xeus got released, and requires nlohmann_json to be 3.12.0 . Until this PR is in the ci will fail
(see https://github.com/compiler-research/xeus-cpp/actions/runs/14604191183/job/40969335106#step:5:97).

Fixes # (issue)

Type of change

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• Bug fix
• New feature
• Requires documentation updates

Testing

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Checklist

• I have read the contribution guide recently

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.27%. Comparing base (4e44614) to head (75d9476).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #566   +/-   ##
=======================================
  Coverage   76.27%   76.27%           
=======================================
  Files           9        9           
  Lines        3650     3650           
=======================================
  Hits         2784     2784           
  Misses        866      866           

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[anutosh491]

Please make the same changes you made on xeus-cpp's side here too

[mcbarton]

Please make the same changes you made on xeus-cpp's side here too

Done

[anutosh491]

Approving feel free to merge once CI passes

check if xeus is 5.2.1 , xeus-lite is 3.1.0 too