Fix last modified project data timestamp

In !382 (comment 540911) @mwolff wrote:

the lastModifiedCMakeFile should be the max timestamp of the single_subdirectory cmake files, so something that depends on the git checkout.

This turns out to be a wrong assumption. Some CMake files are modified during build, for example: kdevelop/build/plugins/cmake/tests/build-single_subdirectory/CMakeFiles/3.24.3/CMakeCXXCompiler.cmake.

The second commit in this merge request attempted to use an earlier timestamp in order to fix Bug 460450: client query file's instead of reply index file's. And this attempted fix makes the test fail reliably.

Complete failed test_cmakefileapi output on my system: test_cmakefileapi-output

@mwolff, did you make this wrong assumption while implementing !360 (merged) too, and so the end result is not entirely intended? Or did you just forget the details by the time of the depends-on-the-git-checkout comment?