CI: comment with rspec-stderr

[runephilosof-abtion]

We should avoid introducing noisy warnings in our test output.
Often the warnings are added by Dependabot upgrades. This way we will notice when it happens.

Make sure that your repository has no output when you merge/cherry-pick this, to avoid adding noise to every PR.

[github-actions]

Rspec stderr output:

warning: parser/current is loading parser/ruby33, which recognizes 3.3.5-compliant syntax, but you are running 3.3.4.
Please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.

[runephilosof-abtion]

This will probably include the annotations from #2328 in the comment...

It looks like it is using stdout, so it won't be a problem.
https://github.com/Drieam/rspec-github/blob/075c231617113d9a05c80513dff17c7c7e27dbae/lib/rspec/github/formatter.rb

[runephilosof-abtion]

Optimally, not only from the rspec step.

Also, it would be nice to fix that warning before merging this.

[runephilosof-abtion]

Maybe I should make the step fail after posting, so that Dependabot PRs are not auto merged.

[runephilosof-abtion]

Oops. All the steps are being run twice (push and pull_request), wasting resources.

[runephilosof-abtion]

I will test it on another project first. Turning it into a draft.

[jeppester]

This is a really interesting idea.
I do wonder if people will learn to ignore this message that same way as normal terminal output.

I wish there was a way to print only the additional error output added by the PR's branch