Upgrade bootstrap js/css

[kanecko]

Issue in rockstor-core: rockstor/rockstor-core#2548

Upgrading bootstrap to at least 4.6 would be nice.
The license and supported browsers look fine.

Reasoning: it has built-in switches. This means we could remove the dependency to external bootstrap-switch library.
Cons: there will be breaking changes (https://getbootstrap.com/docs/4.6/migration/, https://getbootstrap.com/docs/5.2/migration/)

My opinion: since v3.x works fine with latest modern browsers, there is no rush to upgrade.
Therefore it is a nice-to-have, but alas a low-prio task.