Remove legacy GitHub Packages

[potiuk]

This PR removes the legacy GitHub Packages support:

• removes checking for images in Packages/Registry
• removes output informing about the registry
• hard-codes registry to ghcr.io
• Updaes documentation describing the registries

Part of #16555

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[potiuk]

Looks like we are good to merge this one. The ghcr.io switch seems successful.

[potiuk]

Good to go this one

[uranusjr]

Can’t say I understand some of the bash stuff but as far a I can tell this is good to merge.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[potiuk]

Can’t say I understand some of the bash stuff but as far a I can tell this is good to merge.

Yeah. Once we simplify some of that I hope we can finally rewrite it in Python: #12282.

This image building and caching done efficiently for development will now be much easier. Some of the complexity I am removing now was because we used both GitHub Packages (For CI) and DockerHub (for Breeze). That was because we did not want to have additional authentication for DockerHub pushing images from CI (and we ere afraid it won't handle the per-build CI images we have). On the other hand, GitHub Packages did not have the option of "Public" images. Now with ghcr.io which was promoted to GA recently, we have both - built-in authentication using GITHUB_TOKEN and public images, so we can finally switch I think.

I will start gradually replacing parts of the bash scripts with Python soon I think (and invite others to help)

With #16780 we are switching Breeze completely to ghcr.io as well and then managing of images becomes much simpler, it did not make much sense to rewrite all the complexity in Python (even in Python it would be equally complex).