ci: Use GITHUB_OUTPUT envvar instead of set-output command

`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
Signed-off-by: Arun <arun@arun.blog>

.github/workflows/ci.yml

@@ -71,7 +71,7 @@ jobs:
         # no creduce on Windows, so exclude tests needing creduce there
         run: |
           echo RUSTFLAGS=$RUSTFLAGS >> $GITHUB_ENV
-          echo ::set-output name=exclude::${{runner.os == 'Windows' && '--exclude autocxx-reduce --exclude autocxx-gen' || ''}}
+          echo exclude=${{runner.os == 'Windows' && '--exclude autocxx-reduce --exclude autocxx-gen' || ''}} >> "$GITHUB_OUTPUT"
         env:
           # non-linux failures https://github.com/google/autocxx/issues/819
           # beta failing tests https://github.com/google/autocxx/issues/818