[libc++] Update the CI Docker image to Focal

[Mordante]

Interesting this fails with GCC-11 for some format functions. I've seen GCC-11 issues in format before, like
in this https://reviews.llvm.org/D112727#change-pkkDJKXHGfxB file.
Can you disable the tests with

// TODO FMT Evaluate gcc-11 status
// UNSUPPORTED: gcc-11

Then it will automatically be in my TODO list.

[ldionne]

I'm actually going to ship this as-is because without this, we can't even build the Docker image anymore. Let's figure out the CI situation separately.

libcxx/utils/ci/Dockerfile

 # A pre-built version of this image is maintained on DockerHub as ldionne/libcxx-builder.
 # To update the image, rebuild it and push it to ldionne/libcxx-builder (which
 # will obviously only work if you have permission to do so).
 #
 #   $ docker build -t ldionne/libcxx-builder libcxx/utils/ci
 #   $ docker push ldionne/libcxx-builder
 #
 
-FROM ubuntu:bionic
+FROM ubuntu:focal
 
 # Make sure apt-get doesn't try to prompt for stuff like our time zone, etc.
 ENV DEBIAN_FRONTEND=noninteractive
 
 # This dummy command is meant to be edited from time to time, which causes the
 # CI builders to rebuild their copy of the Docker image. This is not a great
 # solution, however without that, the CI builders will keep the same cached
 # Docker image forever.