CI: migrate CleanerX to Build Explorer v2 metadata

[WieszczY85]

What changed

• migrates CleanerX publishing from Build Explorer v1 to /api/v2/builds/ci/cleanerx
• groups Paper and Spigot under one logical cleanerx project
• infers channel from the built JAR name, with experimental/** and experimental-* forcing WIP
• reads Minecraft versions from the platform-specific VersionChecker.kt
• sends compatible_platforms metadata; each CleanerX artifact currently declares its primary platform
• fixes case-sensitive module/path names (CleanerX-Paper, CleanerX-Spigot) in the existing workflow
• keeps PR runs verification-only; artifacts are not uploaded from pull requests

Channel rules

• experimental branch, -WIP, preAlpha, alpha -> wip
• -DEV -> dev
• -SNAPSHOT, beta, RC -> snapshot
• otherwise -> release

Deployment prerequisite

Deploy miniPORTAL Build Explorer 2.0.1 (miniPORTAL-source PR #15) and run its compatible-platform migration before relying on the new metadata. The logical cleanerx project slug must exist before the first post-merge upload.