Fix ESLint 6 support (#7513)

* Remove extends restriction to fix ESLint 6 support

* Attempt to resolve CI errors

Author: ianschmitz

docusaurus/docs/setting-up-your-editor.md

@@ -47,7 +47,7 @@ Note that any rules set to `"error"` will stop the project from building.
 
 There are a few things to remember:
 
-1. You must extend the base config, as removing it could introduce hard-to-find issues.
+1. We highly recommend extending the base config, as removing it could introduce hard-to-find issues.
 1. When working with TypeScript, you'll need to provide an `overrides` object for rules that should _only_ target TypeScript files.
 
 In the below example:

packages/react-scripts/config/webpack.config.js

@@ -347,14 +347,8 @@ module.exports = function(webpackEnv) {
                     // A config couldn't be found.
                   }
 
-                  // We allow overriding the config, only if it extends our config
-                  // (`extends` can be a string or array of strings).
-                  if (
-                    process.env.EXTEND_ESLINT &&
-                    eslintConfig &&
-                    eslintConfig.extends &&
-                    eslintConfig.extends.includes('react-app')
-                  ) {
+                  // We allow overriding the config only if the env variable is set
+                  if (process.env.EXTEND_ESLINT && eslintConfig) {
                     return eslintConfig;
                   } else {
                     return {