Forbid junit-dep from system-rules

[jglick]

Seems to prevent an error in a proprietary plugin when trying to validate using Maven 4.0.0-rc-1:

[ERROR] Failed to execute goal on project xxx: Could not collect dependencies for project xxx:hpi:999999-SNAPSHOT
[ERROR] org.eclipse.aether.resolution.VersionRangeResolutionException: No versions available for junit:junit-dep:jar:[4.9,) within specified range
[ERROR] 	Caused by: No versions available for junit:junit-dep:jar:[4.9,) within specified range
[ERROR] : Failed to collect dependencies at io.jenkins.configuration-as-code:test-harness:jar:1897.v79281e066ea_7 -> com.github.stefanbirkner:system-rules:jar:1.19.0 -> junit:junit-dep:jar:[4.9,)

Tried without success to reproduce in an OSS POM; the trigger condition seems to relate to use of a proprietary <repository> with snapshots enabled, which I guess causes the resolver to attempt to search for a valid junit-dep version? Anyway we definitely do not want this transitive dep, and removing it fixes this error. (I still have other problems with some repos, even this one, using Maven 4.)