ci: replace defunct endaft/action-changelog with action-gh-release auto-notes

endaft/action-changelog has been removed from GitHub (returns 404 on
the action checkout step), which broke every release workflow run
since rc4. Switches the Release step to use
softprops/action-gh-release@v2's built-in generate_release_notes: true
option, which pulls the autogenerated PR/commit summary from GitHub's
release-notes API. Same outcome, no dependency on a deleted third-party
action.