Add New Settings for RKE2/K3S Default Versions

[jiaqiluo]

Issue: #36827

Problem

The default rke2/k3s version should be configurable per rancher version.
Both the default rke2/k3s version and rancher version need to be a range so we do not need to add a new entry each time when we release a new rancher version.

Solution

Rancher will get the version ranges from the channelserver, calculate the default versions for the current rancher version, and expose them as new Settings items:

• rke2-default-version
• k3s-default-version

And they are available under the /v1/management.cattle.io.settings endpoint automatically.

Dependency

We need to add the support for appDefaults to the channelsesrver : rancher/channelserver#17

Testing

• the correct default versions show up in the settings after rancher runs up
• the correct default versions show up in the settings when the external KDM is not accessible or out-of-date. This is to simulate the airgap environment.
• the default versions are updated correctly after corresponding values are updated in the KDM

Note

To make the CI pass, this PR replaces the rancher/channelserver module with my fork. It will be switched back once the above PR to the channel server is merged.
Update: the PR in rancher/channelsever was merged and we switched back to use the latest commit for it.

[jiaqiluo]

Hi @thedadams @paynejacob , this PR is ready for your final review.
The changes since your last approval are:

• switch from my fork to rancher/channelserver in go.mod
• squash and rebase the PR