[RM] Remove containerd from mantic-proposed

I'd like to request the removal of src:containerd version 1.7.2-0ubuntu2 and its binaries from mantic-proposed.

As a continuous effort of the Server team, we have been trying to keep runc, containerd and docker.io up-to-date across all supported releases. However, we have been facing issues when updating those library packages (-dev packages) because we already have a bunch of reverse dependencies in the archive. The solution to that for docker.io was to split it into src:docker.io (providing the library package) and src:docker.io-app (providing the applications), more info in LP: #2022390. It is done in mantic and the idea was to do the same for containerd and runc in future updates to see how it goes with docker first. However, the current update of containerd to version 1.7.2 has been challenging, because its library package (-dev package) requires a version bump of some dependencies to make it work (it is not using the vendor code). Initially, I thought it would be OK to update the dependencies but it is becoming a rabbit hole, it requires a major bump of some packages and a backport of some upstream patches for src:docker.io (~ in sync with Debian) to support this new version. This would be really hard to be backported to supported releases via SRU.

My idea here is to remove the current containerd version we have in mantic-proposed, which would leave us with containerd/1.6.12-0ubuntu3 in the mantic release pocket, then add a task for containerd in LP: #2022390, create a new source package called containerd-app to ship just the application (version 1.7.2) and "sync" the -dev package from Debian (version 1.6.20~ds1-1+b1). With that, we would follow the same pattern of docker.io, and the backport of the application would be more straightforward from now on.