Upgrade bundled libcurl to 8.21.0 in librdkafka/confluent-kafka artifacts

## Summary

Our dependency scan reports the following dependency chain:

confluent-kafka 2.14.2 → librdkafka 2.14.2 → Source/autoconf builds: libcurl 8.20.0, vcpkg-based builds: libcurl 8.19.0

The bundled curl/libcurl component is flagged for these CVEs:

CVE-2026-10536, CVE-2026-11564, CVE-2026-11856, CVE-2026-8924,
CVE-2026-8925, CVE-2026-8926, CVE-2026-8927, CVE-2026-9079, CVE-2026-8458

We are using:

- confluent-kafka==2.14.2
- librdkafka==2.14.2

Please confirm whether the distributed confluent-kafka/librdkafka artifacts bundle or depend on this curl/libcurl version.

## Requested action
- Provide the minimum fixed version or recommended mitigation.

## References

- Similar dependency issue: https://github.com/confluentinc/confluent-kafka-python/issues/2282
- curl security advisories: https://curl.se/docs/security.html
- curl 8.21.0 release/advisories: https://curl.se/mail/archive-2026-06/0012.html