3.0 RFC/epic: Drop support for IE11, Ember <2.4, and more

Motivation

There are several costs to ember-table supporting legacy platforms, and we're ready for a major release.

• Contributors keep stumbling on the legacy use of sendAction in this codebase (most recently at https://github.com/Addepar/ember-table/pull/818). Today we cannot remove sendAction since it is required for Ember 1.12 and this library supports Ember 1.12.
• Additionally this library supports IE11 despite that browser being extremely uncommon in modern usage. Indeed at Addepar we no longer have a need to support IE11, and I think we're one of the last.
• We want to land https://github.com/Addepar/ember-table/pull/814, but may require a breaking change to the DOM (a new <div>)
• We want to drop support for legacy Node, < v10
• As part of an effort to update the ember-cli-sass version used here and elsewhere we make changes in https://github.com/Addepar/addepar-style-toolbox/pull/38 which may alter how the SASS of an application works (no longer would an example app have a patched @import to import only once, instead an application should provide that functionality itself if desired).

The plan & progress

The repo now has a branch 3.0-beta, in draft state at https://github.com/Addepar/ember-table/pull/831. We will land patches for an eventual 3.0 release into that branch, and will make incremental beta releases along the way. Already you can find v3.0.0-4 released via NPM.

I propose a list of changes before a 3.0 final is declared. The task list is:

• Drop Ember < 2.4 from ember-try.js https://github.com/Addepar/ember-table/blob/master/config/ember-try.js#L13
• Drop usage of sendAction, instead call passed-in functions directly. Basically https://github.com/Addepar/ember-table/pull/825 but only the non-Ember-1.13 codepaths. https://github.com/Addepar/ember-table/pull/801 https://github.com/Addepar/ember-table/pull/860 should be able to be closed.
• Drop support for IE11 in https://github.com/html-next/vertical-collection (as audited by @twokul and @mixonic on 2020-12-22 we consider this done)
• Drop support for IE11 in this repo (for example https://github.com/Addepar/ember-table/blob/70d09600c0032f19cad1f62c050cde2d993e8365/addon/-private/sticky/legacy-sticky-polyfill.js#L4)
• Land https://github.com/Addepar/ember-table/pull/814 into the 3.0 branch
• Drop support for legacy Node in the package.json
• Land & release vertical-collection 2.0 https://github.com/html-next/vertical-collection/pull/321
• Release ember-raf-scheduler@0.2.0; https://github.com/Addepar/ember-table/pull/870
• Bump ember-cli-sass
• Resolve any open 3.0 labeled issues or pull requests: Issues & PRs filtered here.
• Cut a major version of ember-table

Start with the 3.0-beta branch and review the remaining unchecked items in this issue, especially vertical-collection 2.0, open 3.0 issues and pull requests, and the major release. The mentioned entry points include config/ember-try.js, package.json, and addon/-private/sticky/legacy-sticky-polyfill.js. Done means the remaining 3.0 work is resolved and a major ember-table release is cut.