Testing: Remove enzyme completely (#44494)

* Remove enzyme completely

* Add CHANGELOG entry

* Document how to use enzyme

* Link from CHANGELOG to README

* Clarify relationship between RTL and React 18 as rationale for removing enzyme

* Remove underscore usage completely

docs/contributors/code/testing-overview.md

@@ -21,7 +21,7 @@ When writing tests consider the following:
 
 Tests for JavaScript use [Jest](https://jestjs.io/) as the test runner and its API for [globals](https://jestjs.io/docs/en/api.html) (`describe`, `test`, `beforeEach` and so on) [assertions](https://jestjs.io/docs/en/expect.html), [mocks](https://jestjs.io/docs/en/mock-functions.html), [spies](https://jestjs.io/docs/en/jest-object.html#jestspyonobject-methodname) and [mock functions](https://jestjs.io/docs/en/mock-function-api.html). If needed, you can also use [React Testing Library](https://testing-library.com/docs/react-testing-library/intro) for React component testing.
 
-_It should be noted that in the past, React components were unit tested with [Enzyme](https://github.com/airbnb/enzyme). However, React Testing Library (RTL) should be used for new tests instead, and over time old tests should be refactored to use RTL too (typically when working on code that touches an old test)._
+_It should be noted that in the past, React components were unit tested with [Enzyme](https://github.com/airbnb/enzyme). However, React Testing Library (RTL) is now used for all existing and new tests instead._
 
 Assuming you've followed the [instructions](/docs/contributors/code/getting-started-with-code-contribution.md) to install Node and project dependencies, tests can be run from the command-line with NPM: