Update github release note body to be able to be parsed correctly

[arpxspace]

RELEASE_NOTES_BODY currently looks like:

## What's Changed
<list of changes>

## New Contributors (this part is optional)
<content here>

**Full Changelog**: https://github.com/SAFE-Stack/SAFE-template/compare/5.0.9...5.1.0

This format doesn't seem to be consistent with what is on RELEASE_NOTES.md and causes a parsing issue from Fake. Will need to ensure that the release notes body is formatted whereby it just has the <list of changes>.