CI: switch to Ubuntu 20.10 to have Qt >= 5.14
CI: switch to Ubuntu 20.10 to have Qt >= 5.14

KDE Frameworks requires Qt 5.14 already so we should adopt that. This
will also save quite a few #ifdefs when migrating to Qt 6.

KDE/qca,KDE/qca,KDE/qca,KDE/qca