Update catch2 version across all exercises

[ahans]

The Catch2 version used here is different from what the test runner uses. To add to the chaos, for complex-numbers #995 adds the test runner's version, because for the matcher used there, v2.13.6 and v3.4.0 are incompatible. To clean this up, we should use a single version everywhere. Concretely this means:

• update all exercises to at least Catch2 3.4.0 (it could make sense to pick something newer, but then the test runner and the local repo should be updated more or less simultaneously)
• remove the special handling from complex-numbers

It could also make sense to introduce some tooling that automates the process of copying the Catch2 files into the exercises' directories.