Drop IE11 support

[lpranam]

This is an Easy Hack. Potential mentors: @lpranam

Detailed description and rationale

As Microsoft is dropping support for IE11: https://docs.microsoft.com/en-us/lifecycle/announcements/m365-ie11-microsoft-edge-legacy We should probably also update our baseline for Chrome, FF, drop legacy Edge (ie. pre Chromium Edge) at the same time; versions TBD.

Give detailed description of the job to be done, preferably along with its rationale We need to:

• [ ] update our documentation / Product Description to match the new versions
• [ ] update eslint versions
• [ ] drop legacy polyfills
• [ ] drop horrible Clipboard.js complexity needed for IE11
• [ ] detect and warn when you launch in IE11 and/or legacy-Edge and offer a friendly warning/upgrade notification built into the loleaflet.html and test that.
• [ ] other things I forgot (?).

Additional notes

Good browser support data here: https://gs.statcounter.com/browser-market-share#monthly-201805-202102