The changelog update script now adds a release header if it doesn't find one already that matches the current release candidate version.

Relates to #10752

Manual testing steps:

For development changelog updates:

• Run yarn update-changelog
• See that it places changelog entries under 'Current Develop Branch'

For RC changelog updates with existing release header:

• Bump the version in app/manifest/_base.json, to simulate an RC-like environment
• Add a new release header to the changelog for the new version
• Run yarn update-changelog
• See that it places changelog entries under the new release header

For updates without a release header:

• Bump the version in app/manifest/_base.json, to simulate an RC-like environment
• Run yarn update-changelog
• See that it places changelog entries under the newly added release header