Fix warning: -Wdeprecated-copy - implement StrutRect::operator=

Summary:
/home/frederik/kdesrc/kde/workspace/kwin/geometry.cpp:156:88: warning: implicitly-declared ‘constexpr KWin::StrutRect& KWin::StrutRect::operator=(const KWin::StrutRect&)’ is deprecated [-Wdeprecated-copy]
  156 |             *strut = StrutRect((*strut).intersected(clientsScreenRect), (*strut).area());
      |                                                                                        ^
In file included from /home/frederik/kdesrc/kde/workspace/kwin/rules.h:32,
                 from /home/frederik/kdesrc/kde/workspace/kwin/client.h:27,
                 from /home/frederik/kdesrc/kde/workspace/kwin/geometry.cpp:30:
/home/frederik/kdesrc/kde/workspace/kwin/utils.h:113:5: note: because ‘KWin::StrutRect’ has user-provided ‘KWin::StrutRect::StrutRect(const KWin::StrutRect&)’
  113 |     StrutRect(const StrutRect& other);
      |     ^~~~~~~~~

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23084