Enable automatically_summarize_mr_review by default

What does this MR do and why?

It's been a couple of days since the feature flag has been globally enabled on production and no issues were found.

This enables the feature flag by default and will be removed when the feature is ready to be released as GA (see discussion: #412796 (comment 1525793642)).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #416706 (closed)