Automatic superseded task cleanup

Octopus Deploy now automatically cancels superseded deployment tasks — queued deployments that are made redundant by a newer one targeting the same project and environment. Only queued (not yet started) tasks are cancelled, and they remain visible in the task log. An optional variant also cancels running tasks paused waiting for external events like pull request merges or manual intervention. The feature is enabled by default for new projects and is configurable per project or runbook.