CompatHelper: bump compat for Convex to 0.16 for package test, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the Convex package from 0.15.4 to 0.15.4, 0.16 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.

[sloede]

@ranocha do you know why no tests are running/queued?

[ranocha]

No idea. I just merged main into this branch, so CI will tell whether it looks okay.

@DanielDoehring Any breaking changes in Convex.jl affecting your workload?

[codecov[bot]]

Codecov Report

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

Project coverage is 92.60%. Comparing base (2da0863) to head (77df414).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1953 +/- ## ========================================== - Coverage 96.11% 92.60% -3.51% ========================================== Files 460 460 Lines 36926 36894 -32 ========================================== - Hits 35490 34164 -1326 - Misses 1436 2730 +1294 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1953/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/Trixi.jl/pull/1953/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `92.60% <ø> (-3.51%)` | :arrow_down: | 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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[ranocha]

@DanielDoehring CI fails due to deprecations (https://github.com/trixi-framework/Trixi.jl/actions/runs/9218674674/job/25362597322?pr=1953#step:7:825) and changes of the results (https://github.com/trixi-framework/Trixi.jl/actions/runs/9218674674/job/25362595206?pr=1953#step:7:1722). Please check whether everything is okay, update the code as required, drop support of the old version of Convex.jl, and ping me when everything is ready to be merged.

[DanielDoehring]

The differences in the Butcher array variables are marginal, so that looks ok.