TSLint is not supported for rush-stack-compiler-4.X packages.

gerald4 

You need to delete the TSlint file 🙂

TSconfig should look like this:

{
  "extends": "./node_modules/@microsoft/rush-stack-compiler-4.5/includes/tsconfig-web.json",
  "compilerOptions": {
    "target": "es5",
    "forceConsistentCasingInFileNames": true,
    "module": "esnext",
    "moduleResolution": "node",
    "jsx": "react",
    "declaration": true,
    "sourceMap": true,
    "experimentalDecorators": true,
    "skipLibCheck": true,
    "outDir": "lib",
    "inlineSources": false,
    "strictNullChecks": false,
    "noUnusedLocals": false,
    "typeRoots": [
      "./node_modules/@types",
      "./node_modules/@microsoft"
    ],
    "types": [
      "webpack-env"
    ],
    "lib": [
      "es5",
      "dom",
      "es2015.collection",
      "es2015.promise"
    ]
  },
  "include": [
    "src/**/*.ts",
    "src/**/*.tsx"
  ]
}

And gulpfile.js should look like this:

'use strict';

const gulp = require('gulp');
const build = require('@microsoft/sp-build-web');

build.addSuppression(`Warning - [sass] The local CSS class 'ms-Grid' is not camelCase and will not be type-safe.`);
build.configureWebpack.mergeConfig({
  additionalConfiguration: (generatedConfiguration) => {
      generatedConfiguration.externals.splice(generatedConfiguration.externals.indexOf('react'), 1);
      generatedConfiguration.externals.splice(generatedConfiguration.externals.indexOf('react-dom'), 1);
      return generatedConfiguration;
  }
});

var getTasks = build.rig.getTasks;
build.rig.getTasks = function () {
  var result = getTasks.call(build.rig);
  result.set('serve', result.get('serve-deprecated'));

  return result;
};

build.initialize(require('gulp'));

Did a blogpost on this for myself so I can keep track on the steps I need to go through as I need to do this here and there:

Link if anyone's interested 🙂 

 https://yourmodernworkplace.com/blog/How-To-Upgrade-A-Old-SPFX-Project-To-Latest-One