Apply --rc automatically on release branches

`auto-changelog validate` has an `--rc` mode which validates that a section for the specified version is present in the changelog, and that the Unreleased section is empty. This is useful (as the name implies) for a release candidate, where a new version is being released. Similarly, `auto-changelog update --rc` will copy over changes in Unreleased to the specified version so as to pass validation.

However, needing to sometimes pass `--rc` and sometimes not complicates how changelogs are validated. It means that that in projects that use `auto-changelog`, the changelog validation step in `yarn lint` and in the `build-lint-test` workflow must check for whether the current branch is a release branch and add `--rc` appropriately. It would be nice not to have to do this, and if instead, `auto-changelog` could detect whether or not it is being run on a release branch and grab the current version from `package.json` automatically.

Start by tracing the `auto-changelog validate` and `auto-changelog update` command entry points and how they currently handle `--rc`; inspect the repository's `package.json` and branch-related logic. Done means release branches automatically use the current package version and RC behavior without requiring `--rc`, while non-release branches retain existing behavior.

Written by the indexing model from the issue text.