Deprecated entities in docbooks

[Nicolas Fella]

Hi,

the KF6 version of kdoctools recently dropped some "long-time deprecated
entities". See
https://invent.kde.org/frameworks/kdoctools/-/commit/fefd1fcce69cf23f2d773dbe059920f110ce4fb2

As a consequence we now have a number of failing application Qt6 CI
builds because their translated docbook still references these now-gone
entities.

Here's a list of usages I found:

knavalbattle: the ru translation uses the &arts; entity

ksnakeduel: the de, pt_BR, pt, uk, et, fr uses the &ksnake; entity. This
should become &ksnakeduel;

There may be more usages than this, especially in apps that don't have
KF6 CI yet.

Please adjust the translations to not use these entities any more.

Thanks

Nico