Fix value_counts for Pandas 2

[caneff]

Two changes here:

1. In 2.0, value_counts has different naming. See https://pandas.pydata.org/docs/whatsnew/v2.0.0.html#value-counts-sets-the-resulting-name-to-count for more details.

2. Fix df.value_counts interaction of subset and dropna, discovered by a new
   doctest failure. Previously we were dropping rows with NA in any column, not
   just the columns of interest if subset was non-empty.

Umbrella issue: #27221

[caneff]

R: @tvalentyn

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[codecov]

Codecov Report

Merging #28500 (4dd9dff) into master (603b517) will decrease coverage by 0.02%.
Report is 98 commits behind head on master.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master   #28500      +/-   ##
==========================================
- Coverage   72.34%   72.33%   -0.02%     
==========================================
  Files         682      683       +1     
  Lines      100536   100754     +218     
==========================================
+ Hits        72737    72877     +140     
- Misses      26221    26299      +78     
  Partials     1578     1578              

Flag	Coverage Δ
python	82.80% <85.71%> (-0.07%)	⬇️

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

Files Changed	Coverage Δ
sdks/python/apache_beam/dataframe/frames.py	95.24% <85.71%> (-0.08%)	⬇️

... and 17 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more