cmake: Don't use deprecated commands to build

Avoid use of deprecated commands when building CMake itself.
Fortunately, the only instance thereof was a use of `subdirs`, which is
replaced by `add_subdirectory` since so long ago that the latter is
already used everywhere else, so there is no danger of this newly
requiring a newer version of CMake to build CMake itself.

Utilities/CMakeLists.txt

@@ -1,7 +1,7 @@
 # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
 # file LICENSE.rst or https://cmake.org/licensing for details.
 
-subdirs(Doxygen)
+add_subdirectory(Doxygen)
 
 if (CMake_DOC_ARTIFACT_PREFIX)
   # Undocumented option for CI usage to reuse already