CompatHelper: bump compat for Convex to 0.15, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the Convex package from 0.14 to 0.14, 0.15.
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-commenter]

Codecov Report

Merging #33 (4ce4a0b) into master (7450802) will increase coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   90.21%   90.57%   +0.36%     
==========================================
  Files          28       28              
  Lines        3935     3640     -295     
==========================================
- Hits         3550     3297     -253     
+ Misses        385      343      -42     

Impacted Files	Coverage Δ
...rc/parameter_estimation/component_loglikelihood.jl	80.00% <0.00%> (-3.34%)	⬇️
src/generate_random_deviates/continuous_rand.jl	82.40% <0.00%> (-0.68%)	⬇️
src/parameter_estimation/fit_glm_vc.jl	95.00% <0.00%> (-0.61%)	⬇️
src/parameter_estimation/fit_gaussian_vc.jl	95.29% <0.00%> (-0.59%)	⬇️
src/parameter_estimation/fit_gaussian_ar_cs.jl	95.65% <0.00%> (-0.39%)	⬇️
src/parameter_estimation/inference_ci.jl	96.89% <0.00%> (-0.31%)	⬇️
src/parameter_estimation/fit_glm_ar_cs.jl	97.18% <0.00%> (-0.31%)	⬇️
...parameter_estimation/update_sigma_and_residuals.jl	81.29% <0.00%> (-0.18%)	⬇️
src/parameter_estimation/initialize_model.jl	98.40% <0.00%> (-0.11%)	⬇️
src/model_interface/AR_interface.jl	100.00% <0.00%> (ø)
... and 17 more

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