Add a function to upgrade instance(s)

### Description

I've just learned today from @rfay that an instance once it is created isn't getting upgraded to the latest version. running `cat /etc/*release*` inside my default instance returned:

```
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=24.04
DISTRIB_CODENAME=noble
DISTRIB_DESCRIPTION="Ubuntu 24.04.1 LTS"
PRETTY_NAME="Ubuntu 24.04.1 LTS"
NAME="Ubuntu"
VERSION_ID="24.04"
VERSION="24.04.1 LTS (Noble Numbat)"
VERSION_CODENAME=noble
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU_CODENAME=noble
LOGO=ubuntu-logo
```

using

```
docker 27.4.1
docker-api 1.47
docker-compose v2.40.3
```

so thing were "a little bit" out of date. per randys recommendation I ran `sudo apt update && sudo apt upgrade -y` upgrading everything to the latest version inside my default instance:

```
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=24.04
DISTRIB_CODENAME=noble
DISTRIB_DESCRIPTION="Ubuntu 24.04.3 LTS"
PRETTY_NAME="Ubuntu 24.04.3 LTS"
NAME="Ubuntu"
VERSION_ID="24.04"
VERSION="24.04.3 LTS (Noble Numbat)"
VERSION_CODENAME=noble
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU_CODENAME=noble
LOGO=ubuntu-logo
```
using
```
docker 29.1.3
docker-api 1.52
docker-compose v2.40.3
```

So I wonder if it would be a reasonable step to surface the option to upgrade an instance more prominently? instead of the manual steps outlined above wrap everything into a command like `limactl upgrade [instance]` or prompt for an upgrade of instance(s) at the end of `limactl start` (for example after lima got updated to a new version). that way it would be ensured that the instances are up to date. as i've said, I was completely unaware of that fact and assumed lima would take care of that under the hood already. so i would guess that more people would be unaware of that detail?