chore: Bump node version used on GitHub

- While [adding trivial change the the code](#365) I bumped into CI being red from various reasons [last one](https://github.com/rwjblue/ember-cli-content-security-policy/actions/runs/8820844298/job/24215355798?pr=365) ended with a hard error on:
```
 [3/5] Fetching packages...
error [email protected]: The engine "node" is incompatible with this module. Expected version "16.* || 18.* || >= 20". Got "12.22.12"
error Found incompatible module.
```
- I tried to reproduce locally, but `[email protected]` is surprisingly not trivial to install on modern OSX without breaking my other projects.
- I tried to consult the [node support](https://github.com/ember-cli/ember-cli/blob/master/docs/node-support.md) matrix for EmberJS and since `16.x` is supported through `3.28.0 - 5.3.0`, it seems like a safe bet.

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ on:
   pull_request:
 
 env:
-  NODE_VERSION: '12.x'
+  NODE_VERSION: '16.x'
 
 jobs:
   lint: