Migrate CI into GitHub workflows

[vdemeester]

trafficstars

Changes

• .github/workflows: update codeql job
• .github/workflows: add slack and retest workflows
• .github/workflows: add a ci workflow…

Closes #2452

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you review them:

• [ ] Includes tests (if functionality changed/added)
• [ ] Run the code checkers with make check
• [ ] Regenerate the manpages, docs and go formatting with make generated
• [x] Commit messages follow commit message best practices

See the contribution guide for more details.

Release Notes

NONE