Update release-ondemand.yml - add autogenerated github release notes

[jessebot]

This sets the --generate-release-notes flag for the chart releaser cr command, which according to the help is:

Whether to automatically generate the name and body for this release. See https://docs.github.com/en/rest/releases/releases`

We use it over at nextcloud/helm and here's an example release it generates:

https://github.com/nextcloud/helm/releases/tag/nextcloud-6.0.1

This is helpful so that there's a bit more detail in tools like RenovateBot and Dependabot that automatically generate PRs for version updates. Here's an example of PR that was generated using github release notes: https://github.com/small-hack/argocd-apps/pull/1195

See also for v1.7.x branch: https://github.com/longhorn/charts/pull/175