Extend [compat] JSON="0.21,1"

[ablaom]

This PR extends the JSON compatibility requirements, and fixes the following breakage: In the early version of JSON, the function JSON.parse would return a Dict{String,Any} but now it returns a custom type JSON.Object{String,Any}. While JSON.Object is an AbstractDict, many type annotations for methods in MLFlowClient.jl expect a Dict. I have mitigated this problem by loosening these type annotations to AbstractDict. (The only reason for having annotations at all appears to be as a guard against improper arguments, but for the internal functions this seems unnecessary. However, not being too familiar with the code base I have kept the annotations.)

I have also added a warning in the Readme against using MLflow > 3.2.0, as I ran into an issue, reported at #76.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.09%. Comparing base (0a92b04) to head (d5c42f3).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
- Coverage   90.39%   90.09%   -0.30%     
==========================================
  Files          21       20       -1     
  Lines         406      414       +8     
==========================================
+ Hits          367      373       +6     
- Misses         39       41       +2     

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[ablaom]

I have been unable to make contact with a regular maintainer and so will merge this in 48 hours, as this is needed downstream and the changes seem to me fairly innocuous. Any review will be greatly appreciated, but let me know ASAP if you want to make one.

cc @pebeto @deyandyankov