fix unintentional drop of character in HISTORY

[davidhewitt]

Change Summary

The [^\n] leading character in the regex to add newlines was being unintentionally thrown away, this PR adjusts the re.sub call to re-emit that character.

Related issue number

N/A

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @sydney-runkle

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[samuelcolvin]

let's remove the body = body.removeprefix("## What's Changed\n") here too, we had to remove it on 2.4 since it has the same heading level as the docs section itself.

Other option is to put it at h3 and all sub items as h4.

[samuelcolvin]

please update.

[davidhewitt]

added a follow-up commit, please review