updatecli: create PRs per active branch and refactor slack notifications

[v1v]

What does this PR do?

• Run on Saturdays, a few hours after VM images are produced.
• Create PRs per active branch instead of using the backport labels.
• Use slack composite action.
• Notify the robots team if something goes wrong with the GH actions.
• Enable auto approval and auto-merge with Mergify [Need to configure the branch protections accordingly]

Why is it important?

A follow-up from #8211

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

$ GITHUB_TOKEN=$(gh auth token) \
  GITHUB_ACTOR=v1v \
  BRANCH_NAME=main updatecli apply \
    --config .ci/updatecli/updatecli-bump-vm-images.yml \
    --values .ci/updatecli/values.d/scm.yml

Produced v1v#7

Then I targeted a different branch

$ GITHUB_TOKEN=$(gh auth token) \
  GITHUB_ACTOR=v1v \
  BRANCH_NAME=main updatecli apply \
    --config .ci/updatecli/updatecli-bump-vm-images.yml \
    --values .ci/updatecli/values.d/scm.yml

Produced v1v#8

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

[pkoutsovasilis]

so some context here, I did talked with @v1v yesterday about this condition author=github-actions[bot] and if it derives only from the github username and if that can be exploited. So @v1v investigated and it looks safe enough, so LGTM

[v1v]

@Mergifyio backport 8.18 8.19 9.0 8.17

[mergify]

backport 8.18 8.19 9.0 8.17

✅ Backports have been created

• #8506 [8.18] (backport #8359) updatecli: create PRs per active branch and refactor slack notifications has been created for branch 8.18
• #8507 [8.19] (backport #8359) updatecli: create PRs per active branch and refactor slack notifications has been created for branch 8.19
• #8508 [9.0] (backport #8359) updatecli: create PRs per active branch and refactor slack notifications has been created for branch 9.0
• #8509 [8.17] (backport #8359) updatecli: create PRs per active branch and refactor slack notifications has been created for branch 8.17