Instead of always placing new changelog entries under the "Current Develop Branch" header, the changelog script now places them under the header for the current release if that release has not yet been tagged.

This eliminates one manual step from the release process.

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:

• 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 throws an error because it cannot find the release header

For testing that it doesn't confuse v10.0.0 and v10.0.0-prefix:

• Bump the version in app/manifest/_base.json to v10.0.0, to simulate an RC-like environment
• Add a header to the changelog for v10.0.0-beta.0
• Run yarn update-changelog
• See that it throws an error because it cannot find the release header, showing that it doesn't confuse v10.0.0 for v10.0.0-beta.0