Refactor theme for Bootstrap 5.1

[DiegoPino]

What?

Barrio Bootstrap 5.5.x is now Bootstrap 5 and our theme + that are simply not going to work well together

Tasks

• Make sure all bootswatch and CDN versions of Bootstrap are using 5.1 and move to e.g https://cdn.jsdelivr.net/npm/bootswatch@5.1.0/dist/minty/bootstrap.min.css
• Open an issue and commit to base theme these changes too since they are still stuck with version 4.5.4 of these but mixing in Bootstrap 5!
• Compare all new base templates with ours and adapt to use the new Flex based classes. https://getbootstrap.com/docs/5.0/utilities/flex/
• Refactor any dropdown and our own templates (and twig templates, e.g Metadata download) that implement these since the new data-toggles are different? https://getbootstrap.com/docs/5.1/components/dropdowns/
• Check all styles.css, custom.css and colors.css for duplication and "non flex respecting" rules

@aksm @alliomeria this will take me a week or more and I will need some help

[DiegoPino]

See https://getbootstrap.com/docs/5.0/migration/ to figure out what needs to change I propose here basically a new structure, we should probably use SASS directly and build the CSS instead of tinkering with custom.css and style.css manually. This also implies re-thinking all templates.