Update version to 56.2.0, add changelog (#8372)

# Which issue does this PR close?

We generally require a GitHub issue to be filed for all bug fixes and
enhancements and this helps us generate change logs for our releases.
You can link an issue to this PR using the GitHub syntax.

- part of #7836

# Rationale for this change

Prepare for the next release
# What changes are included in this PR?

1. Update changelog (see rendered preview here:
https://github.com/alamb/arrow-rs/blob/alamb/prepare_for_56.2.0/CHANGELOG.md)
2. Update version

# Are these changes tested?

By CI
# Are there any user-facing changes?

Author: alamb