There is an Add-on in the marketplace called "Automated Release Notes for JIRA" that is available for Cloud. Have you tried that? https://marketplace.atlassian.com/plugins/amoeboids.releasenotes/cloud/overview
Better PDF Exporter is an app that has been available for Server and Data Center for many years, and it has been released for Jira Cloud just recently.
It can generate any sort of document type output from Jira data, including release notes.
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