ANN: (docker-coq: Bump to Debian 11) & (docker-coq-action: Remove `ocaml_version: "minimal"`)

After the merge of PR coq-community/docker-base#20, the images of the four Docker Hub repos below now all(§) come with Debian 11, as well as with the latest release of dune.3.3.1:

(§) except for coqorg/coq:8.5.3-ocaml-4.02.3 for the time being, because of an opam build issue to be investigated.