ci: Notify on Slack about pre-staging environment

[ppawlowski]

Description

This pull request extends Create fresh app instance for PR workflow and adds a Slack notification once pre-staging environment deployment pipeline completes. Example message: https://flowforgeworkspace.slack.com/archives/C067BD0377F/p1717953055911139 Additionally, it switches tests pipeline to use organization-wide variable for github-slack users mapping.

Related Issue(s)

closes https://github.com/FlowFuse/flowfuse/issues/3998

Checklist

• [x] 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

Labels

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