Update launchfeedback docbook to 5.12

proofread + update
bump date + releaseinfo
adapt to changed gui strings
replace kde with plasma

Differential Revision: https://phabricator.kde.org/D11711

(cherry picked from commit 71a6081d11e92b7fd13a5a96fc1e521431c1e02a)

backport to 5.12 LTS