kafka: remove legacy Sarama implementation and tests

What to build

After #4167 completes the Kafka Sink migration to franz-go, remove the remaining legacy Sarama implementation, adapter-only tests, admin abstractions, generated mocks, and test tooling in one dedicated cleanup PR. Preserve the active franz-go Kafka Sink behavior, user-visible configuration semantics, error semantics, and resource-close behavior while moving equivalent coverage to the franz-go adapter where needed.

Acceptance criteria

• #4167 is merged and the active Kafka Sink production path uses franz-go.
• Remove legacy Sarama producer and admin implementations, along with interfaces used only by those implementations.
• Remove generated mocks and mock-generation entries that exist only for the legacy Sarama path; make generate_mock produces only expected changes.
• Remove obsolete Sarama adapter tests for compression, Kafka version selection, retry, timeout, SCRAM setup, and other Sarama-only configuration details.
• Preserve equivalent user-visible Kafka configuration coverage in the franz-go adapter, including existing error and resource-close semantics.
• Kafka Sink production code, tests, and test tooling no longer import or reference Sarama.
• make check passes.
• Race-enabled tests pass for the security, Kafka Sink, and downstream Kafka adapter packages.
• Relevant Kafka integration tests pass.

Blocked by

• #4167

First verify that #4167 is merged and inspect the active franz-go Kafka Sink path. Then locate the legacy Sarama producer and admin implementations, adapter-only tests, generated mocks, and mock-generation entries, using existing Kafka configuration and integration tests as the coverage map. Done means no Sarama references remain, equivalent error and resource-close behavior is covered, generation and checks pass, and the specified race and integration tests succeed.