It is possible to add new headers for nonexistent releases

[This commit in `core`](https://github.com/MetaMask/core/commit/742e7d83ca4470cee04d3e5cede0f7eaf6744c34) added a header for an upcoming release even though that release had not been cut yet. This shouldn't be possible.

## Acceptance Criteria

- When running `auto-changelog validate` in a repo against a changelog which has a header for a release for which there is no matching tag, a validation error should be produced.

Start by running `auto-changelog validate` against a changelog containing a release header without a matching tag, then trace the validation entry point and its existing tests. Done means validation reports an error for that header while preserving current behavior for headers with matching tags.

Written by the indexing model from the issue text.