Update comparison with Vertico

[okamsn]

As discussed in #588, Vertico has changed much recently. The comparison in the README needs to be updated.

User Minad said:

Yes, it makes sense to update the Vertico/Selectrum comparison. Note that Vertico has changed a lot recently, so removing that sentence will not lead to a description which correctly reflects the status quo. So maybe updating the description should be done in a separate PR. Vertico acquired many more features due to its extensions. The main issue with Selectrum is that it doesn't implement support for dynamic completion tables correctly and has multiple other compatibility issues (#114, #481, #532). This was the main reason why I started to experiment with a new implementation, which then turned into Vertico. See also https://github.com/minad/vertico/#alternatives. (EDIT: For more background see also the thread in #532, where we discussed about dynamic completion tables and I started with a prototype implementation originally called minicomp.)