Upgrade from Bootstrap 3 to Bootstrap 5

Found some buttons that were not aligned in the UI, and when I searched for the version of Bootstrap, realized we are using version 3 for `cwlviewer`. Bootstrap 5 was released some months and it has better accessibility, CSS grid & flex support.

I'm working on a similar issue [for another web tool](https://github.com/NatLibFi/Skosmos/issues/1045), and I can volunteer to update `cwlviewer` too (especially since `cwlviewer` UI is much simpler). But happy if other contributors/dev join or takeover this task :+1: