KAFKA-20430 Fix getTopicConfig() for connector-specific offset stores

[kaijaytu]

Follow-up fix for #20384.

When internal.topics.automatic.creation.enable is set to false and a
connector-specific offset topic is missing, the error message
incorrectly references the worker-level config offset.storage.topic
instead of the connector-level config offsets.storage.topic. This is
because readWriteStore() and readOnlyStore() create anonymous
subclasses of KafkaOffsetBackingStore that do not override
getTopicConfig().

This patch overrides getTopicConfig() in both factory methods to
return SourceConnectorConfig.OFFSETS_TOPIC_CONFIG
(offsets.storage.topic), so the error message correctly directs users
to the relevant connector configuration.

Changes

• KafkaOffsetBackingStore.java: Override getTopicConfig() in
  readWriteStore() and readOnlyStore() anonymous classes to return the
  connector-level config key
• Added KafkaOffsetBackingStoreGetTopicConfigTest.java to verify the
  fix

Testing

Unit tests verify that:

1. Worker-level KafkaOffsetBackingStore returns offset.storage.topic
   (unchanged behavior)
2. readWriteStore() now returns offsets.storage.topic (was
   incorrectly returning offset.storage.topic)
3. readOnlyStore() now returns offsets.storage.topic (was
   incorrectly returning offset.storage.topic)

Existing tests (KafkaOffsetBackingStoreTest,
ConnectorOffsetBackingStoreTest) pass with no regression.

[kaijaytu]

@chia7712 This addresses the review comment from #20384 regarding the incorrect config key in error messages for connector-specific offset topics.

[m1a2st]

Please also address this comment #20384 (comment)

[kaijaytu]

Addressed. This PR now also fixes the immutable KIP link in upgrade.md as suggested by @m1a2st in #20384.

[kaijaytu]

Note: the error message in KafkaTopicBasedBackingStore also says "before starting the worker", which isn't accurate for connector-specific offset topics (they are checked at connector startup). Should I also fix the error message wording, or is that better addressed in a separate patch?