gradle-report: name UP-TO-DATE as the cause of a stale report

Found by running it: detekt exited 0 while the report predated the run,
because Gradle skipped the task. 'STALE' alone leaves the reader to work out
why nothing changed, so it now says which task was skipped and how to force
it, with --allow-stale to accept an older report deliberately. Same shape as
gradle-test, which learned this first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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