Stop testing playbooks/roles on Ubuntu Xenial (was: zuul-jobs testing at an impasse)

On 2021-09-17 19:35:31 +0000 (+0000), Jeremy Stanley wrote: [...]

D. remove Ubuntu Xenial testing for zuul-jobs in general?

[...]

Following a bit of discussion[*] in Matrix, I've proposed that as a straw man:

https://review.opendev.org/809899

[*] https://meetings.opendev.org/irclogs/%23zuul/%23zuul.2021-09-17.log.html#t20...