Feature: No longer support IE11

[hwillson]

This issue tracks the removal of IE11 support from Apollo Client.

[benjamn]

@hwillson @brainkim Here's an example of a similar plan for Vue, though of course native Proxy support is the most urgent reason for Vue to stop supporting IE11: https://github.com/vuejs/rfcs/blob/master/active-rfcs/0038-vue3-ie11-support.md

[jpvajda]

Now that IE 11 is official retired ☠️ is there any actual work to do here, or can we just close this one out @benjamn @alessbell @hwillson ?

[alessbell]

Looks like this issue is also no longer relevant #8490

Found a few merged PRs that added workarounds for some IE11 issues we probably want to revert:

• Check Symbol.for exists in addition to Symbol before using it. #6324
• Feature detect Symbol support. Use string as a fallback. #5840 (referenced in our CHANGELOG)
• Fix apollo-cache-inmemory in IE11 #4012 (referenced in our CHANGELOG)