Fix legacy convert for pandas 2.0

[martinkim0]

• Closes Legacy model convert fails because of pandas 2 #2211
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest docs/release_notes/index.md file if fixing a bug or adding a new feature.
• If the changes are patches for a version, I have added the on-merge: backport to x.x.x label.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (7122eb9) 89.08% compared to head (a1af994) 89.10%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2212      +/-   ##
==========================================
+ Coverage   89.08%   89.10%   +0.01%     
==========================================
  Files         145      145              
  Lines       11810    11809       -1     
==========================================
+ Hits        10521    10522       +1     
+ Misses       1289     1287       -2     

Files Changed	Coverage Δ
scvi/model/base/_utils.py	89.79% <100.00%> (-0.07%)	⬇️

... and 2 files with indirect coverage changes

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[martinkim0]

test linux cpu prerelease

test macos cpu

test windows cpu