CI: ubuntu-toolchain-r/test repository will be removed (#14641)

CI: ubuntu-toolchain-r/test repository will be removed (#14641)

* CI: ubuntu-toolchain-r/test repository will be removed

The ubuntu-toolchain-r/test PPA will be removed from Ubuntu images.
The images rollout process will start on May 6 and take 3-4 days.
Mitigation ways

The repository can still be added manually in runtime by calling to following commands:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
sudo apt-get update -y

* fixup: add a note what this repo is for

Signed-off-by: Philip H <47042125+pheiduck@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>