CTest: Remove use of deprecated subdirs command
This command was used in the generated CTestTestfile.cmake files.
Both the add_subdirectory and subdirs command are overwritten in a CTest context and the only difference is that subdirs is a variadic function, otherwise their implementation are essentially identical. They both call the ReadDependentFile() method on the makefile.
Fixes: #22921
Topic-rename: ctest-no-subdirs