Port usage of kcmshell5 to kcmshell6

Just two instances:
- in useractions for the menu action "Configure Window Manager"
- in the docbook about the key shortcuts

doc/windowspecific/index.docbook

@@ -471,7 +471,7 @@ and <guibutton>Move Down</guibutton> buttons effects on how they are applied.</p
   </varlistentry>
   <varlistentry><term><emphasis role="bold">Ignore global shortcuts</emphasis></term>
     <listitem>
-    <para>Toggle whether to ignore global shortcuts (as defined by <menuchoice><guimenu>System Settings</guimenu><guisubmenu>Shortcuts and Gestures</guisubmenu><guimenuitem>Global Shortcuts</guimenuitem></menuchoice> or by running <varname>kcmshell5 keys</varname> in <guilabel>konsole</guilabel>) while the window is active.</para>
+    <para>Toggle whether to ignore global shortcuts (as defined by <menuchoice><guimenu>System Settings</guimenu><guisubmenu>Shortcuts and Gestures</guisubmenu><guimenuitem>Global Shortcuts</guimenuitem></menuchoice> or by running <varname>kcmshell6 keys</varname> in <guilabel>konsole</guilabel>) while the window is active.</para>
      </listitem>
   </varlistentry>
   <varlistentry><term><emphasis role="bold">Closeable</emphasis></term>

src/useractions.cpp

@@ -304,11 +304,11 @@ void UserActionsMenu::init()
             QProcess *p = new QProcess(this);
             p->setArguments(args);
             p->setProcessEnvironment(kwinApp()->processStartupEnvironment());
-            p->setProgram(QStringLiteral("kcmshell5"));
+            p->setProgram(QStringLiteral("kcmshell6"));
             connect(p, static_cast<void (QProcess::*)(int, QProcess::ExitStatus)>(&QProcess::finished), p, &QProcess::deleteLater);
             connect(p, &QProcess::errorOccurred, this, [](QProcess::ProcessError e) {
                 if (e == QProcess::FailedToStart) {
-                    qCDebug(KWIN_CORE) << "Failed to start kcmshell5";
+                    qCDebug(KWIN_CORE) << "Failed to start kcmshell6";
                 }
             });
             p->start();