Rework global.json behavior

(pretty sure there is a ticket for that but didn't find it with github search so please feel free to flag it as duplicate if you have it handy)

### Is your feature request related to a problem? Please describe.
I have a project requiring the sdk net8.0 (similarly 9.0) without a minor specifically.
Using global.json is pretty hard since it requires a specific version whereas we want a compatible version with respect of the version policy (latestMinor for ex).
Most CI will fail if you set 8.0.0 because it doesn't exist but depending the environment you can't set the latest - when the SDK is downloaded from a mirror for example - nor you can assume using 8.0.100 is a good choice.

### Describe the solution you'd like
Idea is to respect the JSON spec, ie if you have a policy, use the version as a baseline so 8.0.0+latestMinor can resolve 8.0.400 for ex.
Ideally support `x` for wildcards would be neat but is not a requirement (8.x, 8.0.x) since it is redundant with the policy.