Upgrade to Express v5.x

[manuelwedler]

Express v5 has been released last year. It improves a few things of Express, most importantly imo, it comes with promise support. For example, we could get rid of the safeHandler with it. While working on API v2, I think it makes sense to also upgrade Express.

It has breaking changes and it seems not straight forward. I already tried quickly to run the server with it, but gave up after half an hour. So, this is not a high prio but still if we have time while working on API v2 it would be nice to have.