CompatHelper: bump compat for Trixi to 0.8 for package test, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the Trixi package from 0.5.29, 0.6, 0.7 to 0.5.29, 0.6, 0.7, 0.8 for package test. 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.27%. Comparing base (fe66f83) to head (33bee27). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #196 +/- ## ======================================= Coverage 80.27% 80.27% ======================================= Files 19 19 Lines 715 715 Branches 50 50 ======================================= Hits 574 574 Misses 137 137 Partials 4 4 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/libtrixi/pull/196/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/trixi-framework/libtrixi/pull/196/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `80.27% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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