ci: Remove image promotion steps

[adamwg]

Description of your changes

We don't build container images for the CLI (yet) and therefore don't have image promotion nix targets, but forgot to remove the image promotion steps from our Promote action.

I have:

• Read and followed Crossplane's contribution process.
• Run ./nix.sh flake check to ensure this PR is ready for review.
  - [ ] Added or updated unit tests.
  - [ ] Linked a PR or a docs tracking issue to document this change.
• Added backport release-x.y labels to auto-backport this PR.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 40d48ae8-28cb-4a55-86f8-4c69f93c0b47

📥 Commits

Reviewing files that changed from the base of the PR and between 3e2485e and 167315d.

📒 Files selected for processing (1)

• .github/workflows/promote.yml

💤 Files with no reviewable changes (1)

• .github/workflows/promote.yml

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📝 Walkthrough

Walkthrough

The promotion workflow is simplified to remove container image promotion to DockerHub, Upbound, and GHCR registries. Environment variables for these registries are removed from the top-level configuration, and all associated login and promotion steps are deleted. The workflow retains only S3 artifact promotion via AWS credentials.

Changes

Promotion Workflow Simplification

Layer / File(s)	Summary
Remove container registry promotion .github/workflows/promote.yml	Environment variables for Docker and Upbound registry access are removed from the workflow top-level env. All container image login and promotion steps targeting DockerHub, Upbound, and GHCR are deleted. The workflow now proceeds directly to the remaining "Promote Artifacts to S3" step, which uses AWS credentials and receives version/channel/pre-release parameters.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: removing image promotion steps from the CI workflow, which aligns perfectly with the changeset.
Description check	✅ Passed	The description is directly related to the changeset, explaining why the image promotion steps are being removed and what was verified before submission.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Breaking Changes	✅ Passed	All files under apis/** and cmd/** are newly added (not modified), with no removal/renaming of public fields/flags, no new required public fields in existing APIs, or removal of existing behavior.
Feature Gate Requirement	✅ Passed	PR removes CI workflow steps (no new experimental features added). No changes to apis/** or CLI behavior requiring feature gates.

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[github-actions]

Successfully created backport PR for release-2.3:

• [Backport release-2.3] ci: Remove image promotion steps #23