fix: remove chain section from forest config

[hanabi1224]

Summary of changes

Changes introduced in this pull request:

• remove chain section from forest config by adding serde(ignore) to the field, as suggested here. The follow-up cleanup will be in fix: remove chain field from struct Config #3608

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[lemmih]

It would be nice to set the chain in the configuration file, but maybe that can come later.

[LesnyRumcajs]

Let's add an entry in the CHANGELOG.

[hanabi1224]

Let's add an entry in the CHANGELOG.

@LesnyRumcajs Done.
(And please re-approve if the change looks OK)