Regularize notation

[jsoref]

The docs https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions clearly indicate ::.

I understand a previous iteration of the environment may have used ##[...], but the current one does not, and messages should reflect how users are supposed to use things today.

[jsoref]

Fwiw, without this change, I have a workflow which used an action which did: echo ::set-output:: and it triggered this:

workflow Unable to process command '::set-output::' successfully.
workflow Required field 'name' is missing in ##[set-output] command.

Which is pretty clearly wrong.

[jsoref]

@fhammerl do i need to do something here?