Release base-files version 10.3+deb10u3co4~v2020.0

Sync to the version on the v2021 branch but ensure that the one for the most recent release is always preferred by using the ~v2020.0 suffix, as discussed on the "Diverging release branches" section of the GitLab packaging workflow documentation, https://gitlab.apertis.org/infrastructure/ci-package-builder#diverging-release-branches

See https://gitlab.apertis.org/pkg/target/base-files/merge_requests/22 for the v2021dev1 related MR.