fix(ci): reduce Maven Central rate limiting

[arnabnandy7]

Summary

• enable Gradle dependency caching in build, test, and Javadoc jobs
• cache dependencies before upgrading the Gradle wrapper for newer Java versions
• limit the integration-test matrix to four concurrent jobs

Context

The current CI matrix starts many Gradle builds concurrently. Recent runs failed while resolving build dependencies because Maven Central returned HTTP 429 responses. Caching dependencies and limiting integration-test concurrency reduces repeated downloads and request bursts.

Fixes #360

[arnabnandy7]

@korniltsev @korniltsev-grafanista FYI

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