When updating the changelog for a release candidate, any unreleased changes are now migrated to the release header.

Generally we don't make a habit of adding changes to the changelog prior to creating a release candidate, but if any are there we certainly don't want them duplicated.

Relates to #10752

Manual testing steps:

• Run yarn update-changelog to add unreleased changes
• See that changes have been added to the "Unreleased" header
• Run yarn update-changelog --rc
• See that it has migrated everything to the v9.3.0 header.
  • Note that any commits without a PR number in the description are duplicated. This is a pre-existing issue, as our duplication detection relies upon the PR number. This will be prevented in the future by mandating GitHub Squash & Merge as the only allowed strategy, which will ensure that a PR number is present in the commit message.