docs: Clarify anndata package status in R section

[rcannood]

Updated the description of the anndata package for R to indicate it is superseded by anndataR.

• Release note not necessary because: trivial docs change

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.60%. Comparing base (d11ccc3) to head (cf11589).
✅ All tests successful. No failed tests found.

Additional details and impacted files

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  Coverage   85.60%   85.60%           
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  Files          49       49           
  Lines        7671     7671           
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  Hits         6567     6567           
  Misses       1104     1104           

[Zethson]

Do you want to keep it or should we even consider not mentioning it?

[rcannood]

If possible I'd prefer to keep it to clear up any confusion between library(anndata) and library(anndataR).