[docs] Update for OpenBSD 7.1.
Make the following documentation changes:
* update last tested version number and emphasize that the process
may work for later platform versions,
* update required packages to emphasize py3 instead of py2.7,
* python3 link is not strictly required to need root.
* update the suggested checkout config file to the stable branch used
in the "upstream" checkout config file, and
* mention some extra flags are now required to properly switch off
Dispatch (since without pr apple/swift-corelibs-libdispatch#559
Dispatch won't compile).
* reinforce building release by default, since the flags likely needed
for debug builds aren't provided in the command snippet.
