chore: bump franz-go to v1.21.6

[vtrubesset]

Summary

This PR bumps franz-go from v1.20.7 to v1.21.6 and updates the related subpackages.

v1.21.2 adds MSK IAM support for custom DNS bootstrap hostnames. When the broker hostname does not match the standard MSK URL format, franz-go uses AWS_REGION or AWS_DEFAULT_REGION instead of failing. See twmb/franz-go#1321.

Connect currently ships franz-go v1.20.7. Upgrading the Connect image alone does not fix MSK IAM with custom DNS until this dependency is bumped.

Changes

• github.com/twmb/franz-go v1.20.7 → v1.21.6
• github.com/twmb/franz-go/pkg/kadm v1.17.2 → v1.18.0
• github.com/twmb/franz-go/pkg/kmsg v1.12.0 → v1.13.1
• github.com/twmb/franz-go/pkg/sr v1.7.0 → v1.8.0

Test plan

• go test ./internal/impl/kafka/...
• CI passes

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