CI: Update perf tests workflow to address deprecation issues

[fallwith]

Our GHA based CI workflow for performance testing is currently warning us of 2 issues:

• 3rd party action adrey/slack-file-upload-action is running on Node v12, but should be on v16+.
• The use of the set-output command is deprecated

We should address these.

[run_performance_tests](https://github.com/newrelic/newrelic-ruby-agent/actions/runs/6166750653/job/16736709122)
The following actions uses node12 which is deprecated and will be forced to run on node16: adrey/slack-file-upload-action@903be3678c88966c762193f06530c39178b44d68. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

[run_performance_tests](https://github.com/newrelic/newrelic-ruby-agent/actions/runs/6166750653/job/16736709122#step:10:7)
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/

[workato-integration]

https://issues.newrelic.com/browse/NR-160282

[workato-integration]

https://new-relic.atlassian.net/browse/NR-160282