Drop support for Internet Explorer and legacy browsers in general

[Jemt]

Internet Explorer (including Microsoft Edge version 12-18) has been dead for years. With this issue, we are officially dropping support for legacy browsers. Fit.UI in its current state will be moved to the legacy branch fitui-v2-legacy where any critical bugs can be fixed. Future development will continue on the master branch and be publishes as version 3.x to the NPM registry.

With increased focus on climate and environment, it makes sense to continue the effort to ensure great compatibility. It is not at all uncommon that people use devices that have passed the 5 or 10 year mark, which especially holds true for expensive devices such as iPads, MacBooks, and Surface tablets/laptops. But we do expect such devices to run the most recent version of the OS and browser available, to function properly with Fit.UI.