Reduce the limit for `commit_message_negative_regex` to 511 characters

Problem

During the migration in #423445 (closed), we discovered a case when one of customers used a regex with 1376 characters: !130394 (comment 1538049553).

To unblock the migration, we temporary increased the length limit for commit_message_negative_regex to 2047 characters.

Proposal

• Contact the customer and reduce the value to fit in 511 characters limit.
• Reduce the limit for commit_message_negative_regex to 511 characters