Replace AutoTuner Maven lookup with offline plugin staleness detection

[parthosa]

Contributes: #2098

Summary

Replace AutoTuner's live Maven latest-release lookup with an offline staleness check. This removes an external network dependency from tuning and cluster recommendation paths, preventing Maven Central HTTP 429 responses while retaining useful plugin update guidance.

Approach

• Continue detecting the existing rapids-4-spark_<scala>-YY.MM.patch artifact name; the repository and product were renamed to NVIDIA cuDF plugin for Apache Spark, but the Maven artifact name is unchanged.
• Treat a plugin release month as potentially outdated only after a two-month grace period.
• Recommend checking the stable cuDF Spark download page without querying Maven or naming an exact latest version.
• Keep the existing missing-JAR and multiple-JAR recommendations.
• Remove the live Maven crawler tests and exact latest-version expectations while keeping the OOM and cluster recommendation tests enabled.

Testing

• cd core && mvn test
• 804 tests passed, 0 failed, 16 ignored.
• Focused AutoTuner and cluster recommendation suites: 101 passed, 0 failed, 7 ignored.

Follow-up

• The fat-wheel direct-download path reported in Maven central throttling in CI #2098 is unchanged.
• WebCrawlerUtil remains in the source tree for a separate cleanup.