poc: replace Bootstrap Vue with Bootstrap 5

[LeoniePeters]

• Removes BV Toast plugin
• Adds Bootstrap 5 stylesheets and js to default, ds4ch and landing layouts
• Creates custom b-toast component
  • resembles BV Toast component, but only with features we currently use
  • uses portal-vue to move DOM to toast container (mainly for stacking toasts)
• New b-toast component used by NewFeatureNotification and PageCookiesWidget

TODO to fully replace BV Toast:

• replace other components
• replace makeToast
• add transition

[github-actions]

JIRA ticket: EC-7200