Upgrade eslint-plugin-matrix-org to 1.2.1

This upgrade came with a number of new lints that needed to be fixed across the code base. Primarily: explicit return types on functions, and explicit visibility modifiers on class members.

.eslintrc.cjs

@@ -1,13 +1,31 @@
+const COPYRIGHT_HEADER = `/*
+Copyright %%CURRENT_YEAR%% New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+`;
+
 module.exports = {
   plugins: ["matrix-org"],
   extends: [
-    "prettier",
     "plugin:matrix-org/react",
     "plugin:matrix-org/a11y",
     "plugin:matrix-org/typescript",
+    "prettier",
   ],
   parserOptions: {
-    ecmaVersion: 2018,
+    ecmaVersion: "latest",
     sourceType: "module",
     project: ["./tsconfig.json"],
   },
@@ -15,27 +33,11 @@ module.exports = {
     browser: true,
     node: true,
   },
-  parserOptions: {
-    ecmaVersion: "latest",
-    sourceType: "module",
-  },
   rules: {
-    "jsx-a11y/media-has-caption": ["off"],
+    "matrix-org/require-copyright-header": ["error", COPYRIGHT_HEADER],
+    "jsx-a11y/media-has-caption": "off",
+    "deprecate/import": "off", // Disabled because it crashes the linter
   },
-  overrides: [
-    {
-      files: ["src/**/*.{ts,tsx}", "test/**/*.{ts,tsx}"],
-      extends: [
-        "plugin:matrix-org/typescript",
-        "plugin:matrix-org/react",
-        "prettier",
-      ],
-      rules: {
-        // We're aiming to convert this code to strict mode
-        "@typescript-eslint/no-non-null-assertion": "off",
-      },
-    },
-  ],
   settings: {
     react: {
       version: "detect",