Replace deprecated set-output command in release.yml

[demiankatz]

The release GitHub action is displaying the following warning:

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/

...we should make necessary updates to address this.