upgrade minimum node-version to >= 18 (#1225)

Drop support for Node 16, which is long past EOL. This will be merged
into the `version-8` branch.

>[!WARNING]
>
>Node 18 is where we're moving to, but that will also be EOL in April,
2025.

Does this require changes to the WorkOS Docs? E.g. the [API
Reference](https://workos.com/docs/reference) or code snippets need
updates.

```
[ ] Yes
```

If yes, link a related docs PR and add a docs maintainer as a reviewer.
Their approval is required.

Author: nicknisi

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node: [16, 18, 20, 22]
+        node: [18, 20, 22]
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4

.nvmrc

@@ -1 +1 @@
-14.21.3
+v18.20.7

package.json

@@ -13,7 +13,7 @@
     "yarn": "1.22.19"
   },
   "engines": {
-    "node": ">=16"
+    "node": ">=18"
   },
   "typings": "lib/index.d.ts",
   "files": [