Choose Action icon and color

[filmaj]

Went to publish a new release and saw this UI:

[zimeg]

Stumbled into some related docs while taking a look at action.yml - stinks that slack isn't allowed 😂

I wonder if we want to continue omitting these values? The marketplace search has a decent branding already IMO. Though the name of this action might be worth revisiting soon...

[zimeg]

This was somewhat updated with some of the changes in slackapi@slack-github-action@v2.0.0! At least the name and description:

For now I think we can close this and keep the default icon, but I'm always open to revisiting later!