Return full `eocs` in `convergence_test`

It would be nice if `convergence_test` would return the full EOCs instead of only the mean values. This would make code [like this](https://github.com/JoshuaLampert/2024_dispersive_shallow_water/blob/983966ca3e332e82d3dc857966c70bc3182dffcd/code/manufactured_solution/convergence_order.jl#L34) a bit shorter and would be consistent with what Trixi.jl v0.15 returns in `convergence_test`, see https://github.com/trixi-framework/Trixi.jl/pull/2753. It is breaking though.