Deprecation of support for Internet Explorer

[autumnfound]

Currently, Internet Explorer represents ~5% traffic on eclipse.org, and less than 1% of total traffic in the past year for many of the other properties that we manage (IoT, Jakarta.EE, openhwgroup). Launching in 2013, this browser still sees regular usage but has become a roadblock to modernizing our framework.

1. No longer supported on many major sites (such as this one, YouTube, and even Google Workspaces)
2. MS is dropping support in August 2021 for Office 365 and has already dropped support for MS Teams
3. Our target for the next theme (BootStrap 5) no longer supports IE
4. IE is a roadblock to modernization (near no support for ES6 and many other modern features)

I believe that the best way to support our users and modernizing our framework would be to deprecate support. We can use a small bit of JS to inject a small toast notification on IE browsers announcing our plan to deprecate support in the near future. This way, we can start the process of modernizing internally soon, and give users time to react to the change in support.