Remove iceberg.connect.group.id config and derive it from consumer group ID

[kumarpritam863]

Problem

The current implementation requires users to set a separate iceberg.connect.group.id configuration that must match the source consumer group ID (consumer.override.group.id / group.id) for the Kafka Connect coordinator to be elected correctly.

This has caused significant confusion and bugs among developers and users since the beginning, including:

• Developers forgetting to set connect.group.id or setting it incorrectly
• Misunderstanding that the two values are actually required to be identical
• Subtle and hard-to-debug coordination issues

Problematic scenario example:

1. Job A is running with consumer.override.group.id = "x-1"
2. Job B is submitted with:
   • consumer.override.group.id = "cg"
   • connect.group.id = "x-1"
3. Both jobs consume from the same topic

Result:
Even though the actual consumer group IDs are different (cg vs x-1) for job B, the coordinator election still happens based on connect.group.id = "x-1". This leads to:

• Wrong group being used for coordination
• Incorrect offset commits
• Potential data loss/duplication or reprocessing
• Very confusing behavior that violates the principle of least surprise

Solution

This PR:

• Removes the connect.group.id configuration completely
• Always derives the Connect coordinator group ID from the actual source consumer group ID (group.id / consumer.override.group.id)
• Renames the internal concept/reference from connectGroupId → sourceConsumerGroupId for clarity (in code/comments where applicable)
• Updates documentation and configuration validation accordingly

Benefits

• Eliminates a entire class of misconfiguration bugs
• Removes a redundant and confusing configuration option
• Makes behavior more predictable and intuitive
• Prevents the problematic scenario described above
• Reduces cognitive load for users and maintainers

Breaking Change

No – Even if "iceberg.connect.group.id" is set, it will be ignored and the correct value will be derived from the consumer group id.

[kumarpritam863]

@bryanck can you please take a look. I think this change is needed. Please let me know you thoughts on this.

[kumarpritam863]

@danielcweeks @ajantha-bhat can we please review this.