replace debian-11 gcc-9-20 and gcc-10-20 by corresponding ubuntu:20.04 toolchains
Summary
Deactivate debian-11 image and all its toolchains
Details
Debian-11 is a work-in-progress unstable distribution. It should not be used for testing our code, since we frequently get unexplainable errors, e.g. #17 (closed) and #18 (closed). This MR replaces all toolchains from the debian-11 image with corresponding toolchains in ubuntu:20.04