Changing our minimum requirements to those of Ubuntu 20.04

[Albert Astals Cid]

Hi, i want to propose to increase our minimum requirements to those of Ubuntu 20.04

Qt 5.12
cmake 3.17
KF5 5.68

There's versions of fedora and opensuse with those so it doesn't seem controversial.

The most controversial would be poppler, that would raise the requirement to 0.86, effectively 0.80 since we don't use any 0.81, 0.82, 0.83, 0.84 or 0.85 API.

This means that none of the opensuse Leap would be able to compile the PDF backend, only opensuse tumbleweed.

How problematic do you think that is?

Keep in mind this would be only for master so for the release happening in August.

Cheers,
  Albert