Run integration tests on Ubuntu 18.04

[pascalberger]

Run integration tests on Ubuntu 18.04 since Ubuntu 16.04 images are no longer available

[jokay]

@pascalberger why not directly update to ubuntu-20.04?

[pascalberger]

@pascalberger why not directly update to ubuntu-20.04?

@jokay The question is if we want to test on the lowest supported environment or the highest. Until now we have done the former.

[jokay]

@jokay The question is if we want to test on the lowest supported environment or the highest. Until now we have done the former.

Ok, just wonder who ever would use Ubuntu 18.04 nowadays but got your point 😉

[pascalberger]

@jokay The question is if we want to test on the lowest supported environment or the highest. Until now we have done the former.

Ok, just wonder who ever would use Ubuntu 18.04 nowadays but got your point 😉

But since .NET Framework / Mono tests will become obsolete, once updated to Cake 2.0, we can instead test against different OS versions later.