Add Slack notification to Check for iOS Release tags github action

[isabelrios]

Based on this issue: #197

I think we should have a way to be notified when this github action fails, otherwise we don't know that milestones are not created due to a failure. The only way is to check if they are in Testrail when they should.
Either in this task or on the others that run after.

cc @mdotb-moz