[ci][cirrus] Replace trusty with bionic/focal in debian_default_toolchain (#3986)

[ci][cirrus] Replace trusty with bionic/focal in debian_default_toolchain (#3986)

Ubuntu 14.04 (Trusty) is out of standard support [1].
``make check`` fails for test_capabilities, as noted in [2].
[3] indicates that the cause is glibc, but that updates are not expected
to the version in trusty.

This PR replaces trusty with Ubuntu 18.04 (Bionic) and 20.04 (Focal).

[1] https://wiki.ubuntu.com/Releases
[2] https://github.com/collectd/collectd/pull/3936
[3] https://github.com/collectd/collectd/pull/3927#issuecomment-953350598

(cherry picked from commit b5d8ad13f4e300a7bfe342d27ad804003fcc9173)

Co-authored-by: Emma Foley <efoley@redhat.com>