Block plugins from adding maven repositories

[ccjernigan]

Is your feature request related to a problem? Please describe.

When Gradle plugins are applied, they can define their own Maven repositories. This could allow dependencies to come from sources that we don't trust.

Describe the solution you'd like

In settings.gradle.kts, configure

repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)

This is likely blocked until this is resolved: https://youtrack.jetbrains.com/issue/KT-52383/Kotlin-Multiplatform-Gradle-Plugin-Unable-to-disable-project-rep