Update changelog rewriting step to match v2+ only changelog notes

[henrymercer]

Fix a mistake I made in 01c7223 where I updated the changelog note but didn't update the changelog rewriting step. We now match on lines starting with - [v2+ only], so we shouldn't have to update the changelog rewriting step for future v2+ only changes.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.