[restructure] Fix source builds and migrate FastStats to 0.27.2 (+ jdk17)

[NoltoxGit]

Summary

• Add the official FastStats Maven repository
• Upgrade FastStats from 0.7.5 to 0.27.2
• Migrate from BukkitMetrics to the new BukkitContext API
• Update the plugin lifecycle to call ready() and shutdown()
• Target Java 17 as required by the latest FastStats SDK
• Remove the stale core module declaration
• Update the Java requirements in the README

Verification

[x] ./gradlew clean build
[x] Confirmed that FastStats is relocated into the final JAR
[x] Confirmed Java 17 compatibility