ci: Use GITHUB_OUTPUT envvar instead of set-output command (#958)

`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

.github/workflows/weekly-pulumi-update.yml

@@ -79,7 +79,7 @@ jobs:
         git update-index -q --refresh
 
         if ! git diff-files --quiet; then 
-        	echo ::set-output name=changes::1 
+        	echo changes=1  >> "$GITHUB_OUTPUT"
         fi
     - name: Initialize submodules
       run: make init_submodules