Extend RFC announcement to include discussions

[timmiesmith]

The PR updates the action that currently announces PR labeled as RFC to the oneDPL Slack channel to include discussions labeled as RFC as well.

[timmiesmith]

I'm using https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#discussion to identify the keywords needed.

[danhoeflinger]

It looks like the action checks github.event.pull_request.labels.*.name, for RFC, but is this the same variable we should be checking for discussions as well? I assume we need a second check or to modify this one to cover github.event.discussion.labels.*.name (guessing the formulation of this).

[timmiesmith]

goofed the rebase. I'm closing this PR and will open another.