ci: Use GITHUB_OUTPUT envvar instead of set-output command (#529)

Summary:
`save-state` and `set-output` commands used in GitHub Actions are deprecated and [GitHub recommends using environment files](https://github.blog/changelog/2023-07-24-github-actions-update-on-save-state-and-set-output-commands/).

This PR updates the usage of `set-output` to `$GITHUB_OUTPUT`

Instructions for envvar usage from GitHub docs:

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

Pull Request resolved: #529

Reviewed By: joe1234wu

Differential Revision: D57408058

Pulled By: wuman

fbshipit-source-id: a2173be292d2ad2f4b62bc4964a0bdac0f26bd01

.github/workflows/docker-publish.yml

@@ -98,7 +98,7 @@ jobs:
 
     - name: Set output
       id: vars
-      run: echo ::set-output name=ref::${GITHUB_REF##*/}
+      run: echo "ref=${GITHUB_REF##*/}" >> $GITHUB_OUTPUT
 
     - name: Pull image for this commit from registry
       run: |