Setting defaults to debian12

[Appu Goundan]

While we recommend using images with explicit debianX version names, we still update tags without setting these explicitly. By default these will now point to debian12 images instead of debian11 images. For example (gcr.io/distroless/base will switch over from gcr.io/distroless/base-debian11 to gcr.io/distroless/base-debian12)

This is scheduled to occur on Oct 20 (in 1 month). See deprecation timelines for debian11 images: https://github.com/GoogleContainerTools/distroless/blob/main/SUPPORT_POLICY.md