Fix deprecated set-output command in release action

[paul121]

Just saw this warning in the v2.4.0 release action run:

Create Release
The set-output command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Turns out that this extended the timeline for deprecating this, so not very urgent, but something we should do. It doesn't seem like we are using the command ourselves so it must be a dependency that we have pinned. Likely softprops/action-gh-release ?