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….78 (cvat-ai#6931)

Co-authored-by: snyk-bot <snyk-bot@snyk.io>
Co-authored-by: Roman Donchenko <roman@cvat.ai>

This automates the first half of the current release process. The second
half will be implemented by another workflow.

The reason why it can't all be done in a single workflow is that it's
useful to let developers inspect what'll go into the release before
actually publishing it, and to apply any last-minute fixes, if
necessary. It also allows CI to complete for the release PR.

To support the new workflow, add a `--set` option to `update_version.py`
that sets the version to a custom value.

This automates the second half of the release process (the first being
automated by `prepare-release.yml`). After this workflow completes, the only
action that should remain for the releaser to do is to merge the
`dev-release-*` pull request. We can't do that as part of this workflow,
because CI has to finish first, and it seems pointless to create another
workflow just to merge 1 PR.

Make the release notes extraction process more sophisticated to work around
GitHub's frustrating handling of line breaks in PR and release
descriptions.