Drop IE11 support #1185
Description
See https://single-spa.js.org/blog/2023/11/27/single-spa-6#browser-support--ie11. A new major release of single-spa will be coming soon, to refactor to typescript. We should drop IE11 support at that time, too.
The assign.js/assign.ts file can be replaced with https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign. The babel/rollup config can change as well. Probably other things to change, too