Unable to define system users within the ui.content module

### Expected Behaviour
It's a common case that one wants to create a system user and deploy it to all the environments with a package.

I should be able to define system users in the ui.content module.

### Actual Behaviour
After adding nodes containing system users, the build fails with errors

### Reproduce Scenario (including but not limited to)

Generate project with the latest archetype e.g:
`mvn -B archetype:generate -D archetypeGroupId=com.adobe.aem -D archetypeArtifactId=aem-project-archetype -D archetypeVersion=27 -D aemVersion=6.5.7 -D appTitle='my app' -D appId=platform -D groupId=my.app -D includeExamples=n`

Inside the ui.content package add a system user node e.g.

#### Logs taken while reproducing problem

[ERROR] ValidationViolation: "jackrabbit-nodetypes: Property 'rep:authorizableId' [String] is not allowed in node with types [rep:SystemUser]: Property is protected!", filePath=jcr_root/home/users/system//.content.xml, nodePath=/home/users/system/, line=6, column=42
[ERROR] ValidationViolation: "jackrabbit-nodetypes: Property 'rep:principalName' [String] is not allowed in node with types [rep:SystemUser]: Property is protected!", filePath=jcr_root/home/users/system//.content.xml, nodePath=/home/users/system/, line=6, column=42

Generate the project with the provided Maven archetype command, add the shown system-user node under the ui.content package, and run the build to reproduce the validation errors. Start with the generated jcr_root/home/users/system//.content.xml and determine what must change so the package accepts the system user; done means the build completes without the protected-property violations.

Written by the indexing model from the issue text.