Change diff* licences to LGPL2+

Be consistent in Kate, while MIT doesn't hurt but lets try to use one
license everywhere.

apps/lib/diff/diffeditor.cpp

 /*
     SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
-    SPDX-License-Identifier: MIT
+    SPDX-License-Identifier: LGPL-2.0-or-later
 */
 #include "diffeditor.h"
 #include "difflinenumarea.h"

apps/lib/diff/diffeditor.h

 /*
     SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
-    SPDX-License-Identifier: MIT
+    SPDX-License-Identifier: LGPL-2.0-or-later
 */
 #ifndef KATE_DIFF_EDITOR
 #define KATE_DIFF_EDITOR

apps/lib/diff/difflinenumarea.cpp

 /*
     SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
-    SPDX-License-Identifier: MIT
+    SPDX-License-Identifier: LGPL-2.0-or-later
 */
 #include "difflinenumarea.h"
 #include "diffeditor.h"

apps/lib/diff/difflinenumarea.h

 /*
     SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
-    SPDX-License-Identifier: MIT
+    SPDX-License-Identifier: LGPL-2.0-or-later
 */
 #ifndef DIFF_LINE_NUM_AREA
 #define DIFF_LINE_NUM_AREA

apps/lib/diff/diffparams.h

 /*
     SPDX-FileCopyrightText: 2022 Waqar Ahmed <waqar.17a@gmail.com>
-    SPDX-License-Identifier: MIT
+    SPDX-License-Identifier: LGPL-2.0-or-later
 */
 #ifndef KATE_DIFF_PARAMS
 #define KATE_DIFF_PARAMS