Relax parsing requirements

Currently the changelog parsing will forgive many formatting issues, but it will fail unless it can find the `Unreleased` header and the `Unreleased` reference link definition. We should try to be more flexible with parsing, to allow validating/updating changelogs that don't exactly match the expected formatting.

Begin by locating the changelog parser and its validation and update paths. Review how the `Unreleased` header and reference link definition are currently required, then identify the expected behavior for changelogs that omit or vary those elements. Done means validation and updating work for accepted formatting variations without breaking existing changelog handling.

Written by the indexing model from the issue text.