meph2-util diff doesn't include mantic from the candidate stream

[Jack Lloyd-Walters]

running `./bin/meph2-util diff https://images.maas.io/ephemeral-v3/candidate https://images.maas.io/ephemeral-v3/stable --new-versions-only --latest-only --promote -o conf/cpc-patch.yaml` gives a diff that includes the deletion of mantic images:

```
...
         - candidate
         - stable
   com.ubuntu.maas:v3:boot:23.04:s390x:ga-23.04:
     versions:
- '20231122':
- labels:
- - candidate
- - stable
- com.ubuntu.maas:v3:boot:23.10:amd64:ga-23.10:
- versions:
- '20231129':
- labels:
- - candidate
- - stable
- com.ubuntu.maas:v3:boot:23.10:amd64:ga-23.10-lowlatency:
- versions:
- '20231129':
- labels:
- - candidate
- - stable
- com.ubuntu.maas:v3:boot:23.10:arm64:ga-23.10:
- versions:
- '20231129':
- labels:
- - candidate
- - stable
- com.ubuntu.maas:v3:boot:23.10:armhf:ga-23.10:
- versions:
- '20231129':
- labels:
- - candidate
- - stable
- com.ubuntu.maas:v3:boot:23.10:ppc64el:ga-23.10:
- versions:
- '20231129':
- labels:
- - candidate
- - stable
- com.ubuntu.maas:v3:boot:23.10:s390x:ga-23.10:
- versions:
- '20231129':
- labels:
- - candidate
- - stable
+ '20231204':
+ labels:
+ - candidate
+ - stable
+ com.ubuntu.maas:v3:boot:24.04:amd64:ga-24.04:
+ labels:
+ - candidate
...
```

Which is odd, given that mantic is in the candidate stream: