WEB: Upgrade Bootstrap from v4.0 to v5.0
Bootstrap has a [new stable version v5.0](https://blog.getbootstrap.com/2021/05/05/bootstrap-5/). I think it is a good idea to upgrade the website to use this new version. It no longer uses jQuery (one less dependency) and new features can be utilized by developers to further improve the website.

Please share your thoughts.

related to issue #41438 and pull request #41447 