Remove support for testing on Ubuntu Bionic 18.04

[ximon18]

It seems that the Incus image no longer exists for bionic. It isn't listed at https://images.linuxcontainers.org/ and ploutos-testing is failing with error:

Run # security.nesting=true is needed to avoid error "Failed to set up mount namespacing: Permission denied" in a
+ sudo incus launch images:ubuntu/bionic/cloud -c security.nesting=true -c environment.DEBIAN_FRONTEND=noninteractive testcon
Launching testcon
Error: Failed instance creation: Failed getting remote image info: Failed getting image: The requested image couldn't be found
Error: Process completed with exit code 1.

See: https://github.com/NLnetLabs/ploutos-testing/actions/runs/10976610432/job/30477578335#step:13:15

To work around this we need to add bionic to the list of no longer supported test environments that are automatically disabled.