Update chef_version to >= 15.3 to require unifed_mode works

[ramereth]

This resolves #154 which was missed in #152

In addition:

• Add 18.04 testing back and remove 21.04
• Fix 20.04 to actually use 20.04
• Use current channel for testing

Signed-off-by: Lance Albertson lance@osuosl.org

[kitchen-porter]

Released as: 3.0.1