Add Ubuntu 20.04 to lts-5.15

Add Ubuntu 20.04 to tqtc/lts-5.15. Currently only Ubuntu 18.04 is supported in lts-5.15.

See comments from Tuukka Turunen on https://bugreports.qt.io/browse/QTCREATORBUG-26167:  " it would also be good to ensure that Qt 5.15 LTS works also with Ubuntu 20.04 and later, since typically users do want to run it with new versions of the supported operating systems (same is true for all platforms, not just Linux). "