Bump to v25.0.0 to pick up removal of `alternative_title`

This picks up changes that have since been made as part of #253.

It's a breaking change, hence the major version bump.
Add changelog for version 25.0.0
Merge pull request #255 from alphagov/bump-version-to-25

Bump to v25.0.0 to pick up removal of `alternative_title`