Bump process version required for job support to 1.6.9

The CPP used to enable it even for version 1.6.8 which
turned out to still be buggy which causes segfaults when
building Cabal with ghc-8.10. See #6986.

This does not fully fix the issue as without job support
build failures can still sporadically occur.

Cabal/src/Distribution/Compat/Process.hs

@@ -26,11 +26,11 @@ import           System.Process (waitForProcess)
 -- in the presence of @exec(3)@ on Windows.
 --
 -- Unfortunately the process job support is badly broken in @process@ releases
--- prior to 1.6.8, so we disable it in these versions, despite the fact that
+-- prior to 1.6.9, so we disable it in these versions, despite the fact that
 -- this means we may see sporatic build failures without jobs.
 enableProcessJobs :: CreateProcess -> CreateProcess
 #ifdef MIN_VERSION_process
-#if MIN_VERSION_process(1,6,8)
+#if MIN_VERSION_process(1,6,9)
 enableProcessJobs cp = cp {Process.use_process_jobs = True}
 #else
 enableProcessJobs cp = cp