Revert "chore: update Node.js to v20 (#22755)"

This reverts removal of support for node v18 in #22755,
and suggests using node v18 instead of node v20 for local dev.

See: yarnpkg/berry#5452

.nvmrc

@@ -1 +1 @@
-v20
+v18

README.md

@@ -14,7 +14,7 @@ To learn how to contribute to the MetaMask project itself, visit our [Internal D
 
 ## Building locally
 
-- Install [Node.js](https://nodejs.org) version 20
+- Install [Node.js](https://nodejs.org) version 18
   - If you are using [nvm](https://github.com/nvm-sh/nvm#installing-and-updating) (recommended) running `nvm use` will automatically choose the right node version for you.
 - Enable Corepack by executing the command `corepack enable` within the metamask-extension project. Corepack is a utility included with Node.js by default. It manages Yarn on a per-project basis, using the version specified by the `packageManager` property in the project's package.json file. Please note that modern releases of [Yarn](https://yarnpkg.com/getting-started/install) are not intended to be installed globally or via npm.
 - Duplicate `.metamaskrc.dist` within the root and rename it to `.metamaskrc` by running `cp .metamaskrc{.dist,}`.

package.json

@@ -443,7 +443,7 @@
     "@types/jest": "^29.1.2",
     "@types/madge": "^5.0.0",
     "@types/mocha": "^10.0.3",
-    "@types/node": "^20",
+    "@types/node": "^18",
     "@types/pify": "^5.0.1",
     "@types/pump": "^1.1.1",
     "@types/react": "^16.9.53",
@@ -586,7 +586,7 @@
     "yargs": "^17.0.1"
   },
   "engines": {
-    "node": ">= 20",
+    "node": ">= 18",
     "yarn": "^4.0.2"
   },
   "lavamoat": {