Upgrade Bootstrap to 5.x

[audiodude]

We are currently on Bootstrap 4.4.1. We received a dependabot alert of moderate severity for this version, and the upgrade path is to 5.x. However, the security alert doesn't affect us because we don't use the "carousel" component.

Either way, it's clear from this incident that security updates are no longer being backported to the 4.x branch. We should update while we have time before it becomes necessary by the next security incident.

[audiodude]

The list of breaking changes in 5.x is quite extensive so this is not a small undertaking.

[Jaifroid]

It all depends on which components are used... We found in kiwix/kiwix-js#1235 that a lot of the "dropped" browsers still work (when the codebase is bundled) for our use case, and the upgrade was quite simple. (But that PR is stalled for other reasons.)

[audiodude]

Thanks for the helpful input @Jaifroid !