Release version 0.8.0 (#38)

Update version to 0.8.0 with
[cargo-release](https://github.com/crate-ci/cargo-release).
Merge this PR to build and publish a new release.

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

GHA: Use `workflow_dispatch` (#31)

This changes the release automation from a model where release PRs are
generated automatically to a model where release PRs are generated
clickops-style with `workflow_dispatch` triggers.