CompatHelper: bump compat for T8code to 0.8, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the T8code package from 0.7.6 to 0.7.6, 0.8.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.46%. Comparing base (5a0811f) to head (fe7ba5e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2982      +/-   ##
==========================================
- Coverage   97.13%   94.46%   -2.67%     
==========================================
  Files         624      624              
  Lines       48439    48433       -6     
==========================================
- Hits        47050    45751    -1299     
- Misses       1389     2682    +1293     

Flag	Coverage Δ
unittests	94.46% <ø> (-2.67%)	⬇️

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[benegee]

To be addressed in #2706