fix!: Rework git-sync CRD to not fail on missing gitFolder

[sbernauer]

Description

If gitFolder was missing the stacklet was basically entirely broken.

For changes see CHANGELOG.md
For CRD change see deploy/helm/airflow-operator/crds/crds.yaml

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

# Author
- [ ] Changes are OpenShift compatible
- [ ] CRD changes approved
- [ ] CRD documentation for all fields, following the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/style-guide).
- [ ] Helm chart can be installed and deployed operator works
- [ ] Integration tests passed (for non trivial changes)
- [ ] Changes need to be "offline" compatible

# Reviewer
- [ ] Code contains useful comments
- [ ] Code contains useful logging statements
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated. Follows the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/style-guide).
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added
- [ ] [Roadmap](https://github.com/orgs/stackabletech/projects/25/views/1) has been updated

[sbernauer]

Please vote for the CRD change on this comment

[sbernauer]

Started https://testing.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/airflow-operator-it-custom/20/

[sbernauer]

Decision was accepted during the daily 2025-03-17

[NickLarsenNZ]

LGTM

[sbernauer]

Release notes test comment :)

airflow-operator/CHANGELOG.md

Lines 15 to 16 in 8a54125

	- BREAKING: The field `.spec.clusterConfig.dagsGitSync[].wait` changed from `uint8` to our human-readable `Duration` struct.
	In case you have used `wait: 20` before, you need to change it to `wait: 20s` ([#596]).