CI jobs fail when Maven Central rate-limits dependency downloads

## Problem

GitHub Actions jobs intermittently fail before compilation or test execution because Maven Central returns HTTP 429 responses while Gradle resolves build dependencies.

Recent failures affected the build, Javadoc, and integration-test workflows. The integration workflow expands into a large matrix, and every job builds `pyroscope.jar` independently inside Docker. This creates a burst of duplicate dependency downloads from the same runner infrastructure.

Example error:

```text
Could not resolve all artifacts for configuration 'classpath'.
Could not GET https://repo.maven.apache.org/maven2/...
Received status code 429 from server: Too Many Requests
```

These failures are infrastructure-related and can make unrelated changes appear broken.

## Expected behavior

CI dependency resolution should remain reliable when the complete build and integration matrices run concurrently.

## Proposed mitigation

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

A longer-term improvement would be to build `pyroscope.jar` once and reuse it across integration jobs instead of running `shadowJar` independently in every Docker build.

## Related

- #359
- Failed main workflow run: https://github.com/grafana/pyroscope-java/actions/runs/32238963961
- Failed integration workflow run: https://github.com/grafana/pyroscope-java/actions/runs/32238964071

Inspect the build, test, Javadoc, and integration workflow definitions first, using the linked failed workflow runs to compare their dependency-resolution behavior. Check the integration matrix and Docker build path for pyroscope.jar; done means the full matrix can run concurrently without recurring Maven Central 429 failures.

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