repo.conf: Switch meta-qt5 layer to hardknott branch to provide Qt 5.15
Qt 5.15 is more mature and stabel than Qt 5.14 as it's an LTS release. As the harkdnott branch is marked explicitely as compatible with dunfell, let's use this instead.
Qt 5.15 is more mature and stabel than Qt 5.14 as it's an LTS release. As the harkdnott branch is marked explicitely as compatible with dunfell, let's use this instead.