CHORE: Deprecate downcast in fillna

[ChengjieLi28]

What do these changes do?

• Deprecate downcast: see DEPR: Deprecate downcast keyword for fillna pandas-dev/pandas#40988 and https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.DataFrameGroupBy.fillna.html
• Deprecated is_categorical_dtype(), use isinstance(obj.dtype, pd.CategoricalDtype) instead (GH 52527)
• Compatible with pandas 2.1.1, mainly for value_counts

Related issue number

Fixes #xxxx

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass

[codecov]

Codecov Report

Merging #705 (b990ca5) into main (bc0c60c) will increase coverage by 11.02%.
The diff coverage is 86.66%.

@@             Coverage Diff             @@
##             main     #705       +/-   ##
===========================================
+ Coverage   82.50%   93.52%   +11.02%     
===========================================
  Files        1025     1025               
  Lines       79377    79374        -3     
  Branches    16443    16443               
===========================================
+ Hits        65491    74237     +8746     
+ Misses      11627     3448     -8179     
+ Partials     2259     1689      -570     

Flag	Coverage Δ
unittests	93.42% <86.66%> (+11.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...ython/xorbits/_mars/dataframe/base/value_counts.py	85.08% <50.00%> (+30.62%)	⬆️
python/xorbits/_mars/dataframe/hash_utils.py	69.23% <66.66%> (+24.40%)	⬆️
python/xorbits/_mars/dataframe/groupby/fill.py	100.00% <100.00%> (+63.95%)	⬆️
python/xorbits/_mars/dataframe/missing/fillna.py	97.31% <100.00%> (+74.38%)	⬆️

... and 165 files with indirect coverage changes

[aresnow1]

LGTM