fix: snakecase to kebab for diffexp-may-be-slow

[tihuan]

The variable diffexp_may_be_slow is set within the python server and used within the javascript client as another variable diffexp-may-be-slow. This PR fixes this divergence and uses a single variable name. The introduced changes have been tested locally and indeed fix the issue that the warning when hovering the DEG button was not shown previously with large datasets.

Original PR: #2678

All credit goes to: @moritzschaefer 🙏

[tihuan]

This PR is currently blocked by #2685, since that PR fixes all the tests that are failing in this PR 👌

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.52%. Comparing base (7bf5add) to head (7d18d8d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
server/data_anndata/anndata_adaptor.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2687   +/-   ##
=======================================
  Coverage   70.52%   70.52%           
=======================================
  Files          96       96           
  Lines        6654     6654           
  Branches      769      769           
=======================================
  Hits         4693     4693           
  Misses       1885     1885           
  Partials       76       76           

Flag	Coverage Δ
frontend	70.52% <50.00%> (ø)
javascript	70.52% <50.00%> (ø)
unitTest	70.52% <50.00%> (ø)

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