[receiver/kafka] Use max_fetch_size instead of default_fetch_size for franz-go consumer

[paulojmdias]

Description

The franz-go Kafka consumer was using default_fetch_size (a Sarama-specific setting) instead of max_fetch_size when configuring kgo.FetchMaxBytes.

This fix ensures the parameter max_fetch_size is used and adds validation to prevent it from being less than min_fetch_size.

The default value for max_fetch_size has been changed from 0 (unlimited) to 1048576 (1 MiB) to maintain backward compatibility with the previous behavior. The default for franz-go is 50MiB.

The default_fetch_size parameter is now marked as deprecated for franz-go and will only be used with the legacy Sarama client.

Link to tracking issue

Fixes #43104

Testing

Added tests to ensure max_fetch_size is equal to or higher than min_fetch_size.

Documentation

Updated documentation referring to the default_fetch_size is deprecated.

[paulojmdias]

@axw IdK if we should set the default value the same as franz-go, which is 50MiB.

At the same time, I added the validation about max_fetch_size being equal to or higher than min_fetch_size, but also want your input about it.

[axw]

Thanks @paulojmdias, looks good - just a few things

[axw]

LGTM! Thanks.

[atoulme]

Approved by codeowner, LGTM, merging.