Time to remove Travis support?

[Mojca Miklavec]

Hi,

If I understand correctly, we'll soon be unable to run any builds on
Travis unless we pay for it (they are gradually moving existing
customers to the new billing plan, so it's just a matter of time):
    https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing

We cannot say that this wasn't expected, so maybe it's time to put the
last nail in that coffin? I don't know if we can expand the OS version
list in any of the other services (GitHub Actions or Azure), but it's
ok even if we don't.

Mojca