Use GITHUB_OUTPUT envvar instead of set-output command as the latter is deprecated

[arunsathiya]

Summary

save-state and set-output commands used in GitHub Actions are deprecated and GitHub recommends using environment files.

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

Testing

• I think the next publish action should successfully go through?

Special notes

I have scanned all repositories owned by slackapi (thanks to gh-clone-org extension from this issue) and there is only one instance of usage.

It's set-output on the npm-publish workflow for deno-slack-sdk repository.

Requirements

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've ran deno task test after making the changes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ca8e418) 98.07% compared to head (34feb7c) 98.07%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #261 +/- ## ======================================= Coverage 98.07% 98.07% ======================================= Files 58 58 Lines 2280 2280 Branches 137 137 ======================================= Hits 2236 2236 Misses 42 42 Partials 2 2 ```

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