fix: Node version requirement >= 20.19

- Fixes #1669

Caused by:

```
require() of ES Module .../.pnpm/@handlebars+parser@file+..+handlebars-parser/node_modules/@handlebars/parser/dist/cjs/index.js not supported.
Instead change the require of index.js in null to a dynamic import() which is available in all CommonJS modules.
```

When using older node in consuming app.

Author: MichalBryxi

package.json

@@ -190,7 +190,7 @@
     }
   },
   "engines": {
-    "node": ">= 18"
+    "node": ">= 20.19"
   },
   "publishConfig": {
     "registry": "https://registry.npmjs.org"

test-apps/new-addon/package.json

@@ -52,7 +52,7 @@
     "webpack": "^5.96.1"
   },
   "engines": {
-    "node": "16.* || >= 18"
+    "node": ">= 20.19"
   },
   "ember": {
     "edition": "octane"

tests/dummy/app/templates/docs/usage.md

@@ -7,6 +7,16 @@ To use AddonDocs, your addon must have a `devDependency` of **Ember v2.8** or hi
 Note that your addon can still support older versions of Ember – it's just that you won't be able to run your AddonDocs documentation site against those older versions. (This means you also cannot write acceptance/application tests against your docs sites on older versions of Ember.)
 
 
+### Node version
+
+Due to [this issue](https://github.com/ember-learn/ember-cli-addon-docs/issues/1669) there is a requirement for consuming app to use [node>=20.19](https://nodejs.org/en/blog/release/v20.19.0/). Otherwise you might get following error during build:
+
+```
+require() of ES Module .../.pnpm/@handlebars+parser@file+..+handlebars-parser/node_modules/@handlebars/parser/dist/cjs/index.js not supported.
+Instead change the require of index.js in null to a dynamic import() which is available in all CommonJS modules.
+```
+
+
 ## Installation
 
 ```sh