libprocessgroup: Switch from gnu++-17 to gnu++-20

This change enables headers like <span>. Inside the <span> header file
file the following guard makes its functionality unavailable when
building with std=gnu++-17:

 #if _LIBCPP_STD_VER > 17
 [ ... ]
 #endif

Bug: 213617178
Change-Id: I5c40708ea196ab112990b5ca6fae9370b75f8752
Signed-off-by: Bart Van Assche <bvanassche@google.com>