feat(airflow): Support 2.7.3, 2.8.1, bump git sync

[maltesander]

Description

• add version 2.7.3, 2.8.1
• remove version 2.6.1
• bump git-sync to 4.2.1

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

- [ ] Changes are OpenShift compatible
- [ ] All added packages (via microdnf or otherwise) have a comment on why they are added
- [ ] Things not downloaded from Red Hat repositories should be mirrored in the Stackable repository and downloaded from there
- [ ] All packages should have (if available) signatures/hashes verified
- [ ] Does your change affect an SBOM? Make sure to update all SBOMs

[adwk67]

lgtm.