Remove "ping" and "ip" from Debian 11+

[tianon]

For historical reasons (related to their usefulness in debugging non-working container networking in container early days before --network container:xxx), Debian 10 and older non-slim images included both ping and ip above minbase, but in 11+ (Bullseye), that will no longer be the case and we will instead be a faithful minbase again 😄

[tianon]

(This is going to create a reproducibility failure for old "unstable" and "testing" builds that will no longer include ip and ping -- this is intentional, and once the CI finishes I'll have a follow-up push to adjust the CI hashes.)

[tianon]

Doh stretch was testing at the time of the CI builds... Maybe this should also still install on epochs less than 2021? (Either that or a hard-coded list of suite names, which is icky and I'd rather avoid.)

[tianon]

Alright, any build with an epoch older than 2021-01-01 will have the old behavior, everything newer will have the new behavior wherein Debian 11+ is a faithful "minbase" in all cases (both slim and non-slim) and Debian 10 and older continue to include ip and ping in non-slim image builds.

[tianon]

(So that means this new script will not recreate the 20210111 builds we published, but I think that's ~fine.)