Release v0.18

### To be included

### To be decided

- [ ] https://github.com/trixi-framework/Trixi.jl/issues/3044
- [ ] https://github.com/trixi-framework/Trixi.jl/pull/2225
- [ ] https://github.com/trixi-framework/Trixi.jl/pull/1255
- If we want this to work smoothly, we will have to break `plot(sol)`
- [ ] https://github.com/trixi-framework/Trixi.jl/issues/1194
- Done as a non-breaking change in https://github.com/trixi-framework/Trixi.jl/pull/3000; we can discuss whether we want to remove constructors made obsolete by this change.
- [ ] https://github.com/trixi-framework/Trixi.jl/issues/2902
- [ ] https://github.com/trixi-framework/Trixi.jl/pull/3202

### Postponed to later

Review the checklist in this issue and follow each linked issue or pull request to understand what is proposed for v0.18. Confirm which items belong in the release and record the unresolved decisions; done means the release contents and postponed work are agreed and the checklist is complete.

Written by the indexing model from the issue text.