Upgrade to a supported Bootstrap version

[vidplace7]

Bootstrap 3.4.x, as included in _static/js/bootstrap.min.js is EOL and no longer receives security updates.

This results in warnings in downstream Debian packaging, build-depends-on-obsolete-bootstrap

The package build-depends on an unmaintained version of bootstrap. Upstream has EOL'ed boostrap 3 and 4 and is not providing public security patches for them. Projects should migrate to bootstrap version 5. Unfortunately, bootstrap 5 is not a drop-in replacement, so a simple switch on the build-dependency may not be enough. The following links provide some guidelines to help developers to migrate:

https://getbootstrap.com/docs/4.6/migration/
https://getbootstrap.com/docs/5.3/migration/

Please upgrade to Bootstrap 5.x