ci: send tests failure Slack notification directly to a user

[ppawlowski]

Description

This pull request changes the way how Slack notifications are sent on failed tests execution.
From now, failed tests executed against a feature branch (pull request) are sent directly to a pull request author via the GitHubBot Slack application. If tests fail against the main branch, notification is sent to the gh-pipelines channel mentioning the last commit author.

Related Issue(s)

https://github.com/FlowFuse/CloudProject/issues/687

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.95%. Comparing base (9f6c480) to head (ae97a8a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7607   +/-   ##
=======================================
  Coverage   76.95%   76.95%           
=======================================
  Files         411      411           
  Lines       21094    21094           
  Branches     5141     5141           
=======================================
  Hits        16233    16233           
  Misses       4861     4861           

Flag	Coverage Δ
backend	76.95% <ø> (ø)

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