deps: upgrade to express 5.1.0

our code is already compliant with v5, and is not affected by the breaking changes described here:
https://expressjs.com/en/guide/migrating-5.html

I ran their codemod command, and it did not find anything – so I also double-checked by manually checking for all of the described changed topics in the migration guide.

with this change npm will still print a warning, due to
`@triliumnext/express-partial-content@"1.0.1"`, which needs to be updated to v5 as well

Author: pano9000