New .NET SDK update policy

Our current SDK update policy implies that our `global.json` always contains the latest version, and CI will install the version based on `global.json`.

This creates problems for various cases:
- on Windows when using Visual Studio, you don't have exact control over the used SDK version
- on Linux, you oftentimes use .NET SDK from system packages and that's not often updated

We need some other system that will us to test against the latest SDKs but won't cause probles in local development experience.

Preliminary plan:
1. Pin up first major build of .NET SDK in `global.json` (e.g. `10.0.100`).
2. Liberally allow for roll forward (minor, major, whatever).
3. Set up the `setup-dotnet` CI action to install a concrete version (e.g. `10.0.400`) instead of the current implicit version from `global.json`.
4. Set up Renovate to auto-update this version on CI, in the action's YAML file.

This way, we'll always use the latest .NET SDK on CI, while allowing for a wide range of SDKs to be used locally when developing Cesium.