Fix GTest fetching with older versions of CMake

I don't think that we really care about the timestamps in this case, but newer (3.24+) versions of CMake complain if the DOWNLOAD_EXTRACT_TIMESTAMP argument (see here) is not set, which is why I added it in !6 (merged). It turned out, however, that it broke the fetching with older versions of CMake, which do not recognize the argument (see !5 (comment 151058)). One way to fix that is to set DOWNLOAD_EXTRACT_TIMESTAMP as the first argument for FetchContent_Declare. This way, older versions of CMake do not think that it is part of the URL_HASH value but seem to simply ignore it. However, I think, it's better not to rely on that behaviour and set the respective policy.