Use release notes generated for stable versions in update modal

Legacy desktop used cms to get release notes for newly offered stable releases of ComfyUI.

We want to reuse those going forward in the update modal, assuming the ComfyUI instance is on the stable updates channel and there is release notes available (since the ComfyUI version is made available as soon as tag is made, there might be no release notes available).

Start at the update modal and trace how newly offered stable releases are checked and displayed. Compare the legacy desktop CMS release-note behavior, then verify that notes appear only for stable-channel updates when available and that updates without notes still work correctly.

Written by the indexing model from the issue text.