chore: remove 'default-distro' from object model 'distribution' object

[ddstreet]

The 'distribution' object is informational, not a configuration object. The correct place for configuration of the project's default distribution version is inside the project configuration.

This places it in the correct object model location in our repo, but doesn't fix the 'azldev' tool.

[azure-pipelines]

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[Copilot]

Pull request overview

Moves the default distribution version into project configuration.

Changes:

• Sets Azure Linux 4.0 Stage 2 as the project default.
• Removes distribution-level default versions.
• Deletes the obsolete Stage 2 override file.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
base/project.toml	Adds the default distribution version.
distro/azurelinux.distro.toml	Removes the Azure Linux default version.
distro/fedora.distro.toml	Removes the Fedora default version.
distro/stage2.toml	Deletes the redundant override.

[reubeno]

These are a part of the existing configurational model; we can't remove them like this.

If you are instead claiming that you don't want them to be, then that's another story.

[ddstreet]

These are a part of the existing configurational model; we can't remove them like this.

Can you elaborate on why you are stating this can't be done?

The documentation for the distros clearly says the default-version field is optional.

The documentation for the project allows specifying the default-distro.

Are you saying that there is something using the default-version defined in the distro object, but that doesn't also look at the default-distro in the project object? Meaning, our configuration cannot be changed even when staying within the documentated format and structure?

I'm just trying to understand what/where this might cause a problem, since it seems like - from the docs as well as our current config - it should be ok? My intention here is to keep the distro objects informational, instead of having any policy included in them. This seemed like an easy, clear-cut simple first step, but maybe I'm missing something?