We should use a LTS version for the ubuntu in the default.yaml

I found we are using the non-LTS version for the Ubuntu.

For most people, they don't know this background:
> LTS or ‘Long Term Support’ releases are published every two years in April. LTS releases are the ‘enterprise grade’ releases of Ubuntu and are used the most. An estimated 95% of all Ubuntu installations are LTS releases.

I viewed the [commits](https://github.com/lima-vm/lima/commits/95324c94d2aacff089bedae7dceaa0e31f8dc823/examples/default.yaml), we often update the version of ubuntu when the ubuntu release a new version. For a new enduser for lima, they would get a default.yaml that contains a non-LTS version of ubuntu if they are using at the year 2021 or 2023.

So, I think we should use the LTS version in the default.yaml

https://github.com/lima-vm/lima/blob/95324c94d2aacff089bedae7dceaa0e31f8dc823/examples/default.yaml#L27
https://ubuntu.com/about/release-cycle