Changes

• Added a few extra CMake variables to capture the version info of kdecoration

• Slight adjustments to the darklydecoration code so that If the version >=6.5.0 then continue to use all the existing radius code introduced for the rounded bottom corners. On Kubuntu it will be skipped since it is not supported in lower versions.

On Kubuntu shown below, the option for rounded bottom corners is disabled by default.

• Also aligned the decoration/CMakeLists.txt moving some of the libraries to private, matching upstream:

https://invent.kde.org/plasma/breeze/-/blob/master/kdecoration/CMakeLists.txt#L26

• Removed include(GNUInstallDirs) from CMakeLists.txt as this was showing the following warnings:

CMake Warning at /usr/share/ECM/kde-modules/KDEInstallDirsCommon.cmake:169 (message):
  KDE_INSTALL_SBINDIR got its value from CMAKE_INSTALL_SBINDIR.  In most
  cases this is unintended, check if you included GNUInstallDirs before
  KDEInstallDirs.  Some third party modules include GNUInstallDirs too so eg.
  find_package(Qt6 ...) is equivalent to include(GNUInstallDirs).  If you set
  CMAKE_INSTALL_SBINDIR deliberately before including KDEInstallDirs its
  recommended to use KDE_INSTALL_SBINDIR instead to suppress this message.
Call Stack (most recent call first):
  /usr/share/ECM/kde-modules/KDEInstallDirsCommon.cmake:258 (_define_relative)
  /usr/share/ECM/kde-modules/KDEInstallDirs5.cmake:242 (include)
  CMakeLists.txt:56 (include)
  CMakeLists.txt:101 (build_qt5)


CMake Warning at /usr/share/ECM/kde-modules/KDEInstallDirsCommon.cmake:169 (message):
  KDE_INSTALL_LIBDIR got its value from CMAKE_INSTALL_LIBDIR.  In most cases
  this is unintended, check if you included GNUInstallDirs before
  KDEInstallDirs.  Some third party modules include GNUInstallDirs too so eg.
  find_package(Qt6 ...) is equivalent to include(GNUInstallDirs).  If you set
  CMAKE_INSTALL_LIBDIR deliberately before including KDEInstallDirs its
  recommended to use KDE_INSTALL_LIBDIR instead to suppress this message.
Call Stack (most recent call first):
  /usr/share/ECM/kde-modules/KDEInstallDirsCommon.cmake:261 (_define_relative)
  /usr/share/ECM/kde-modules/KDEInstallDirs5.cmake:242 (include)
  CMakeLists.txt:56 (include)
  CMakeLists.txt:101 (build_qt5)

and aligned with upstream who don't have that:

https://invent.kde.org/plasma/breeze/-/blob/master/CMakeLists.txt?ref_type=heads#L13