Update to 12.0-RELEASE schedule

The 12.0-RELEASE schedule has been updated, adding BETA4 to the schedule
in order to provide additional testing for changes that fix a boot time
issue, in addition to allowing more time for packages for arm and arm64
to finish building, before branching releng/12.0 and starting the RC 
phase of the cycle.  (Note, the change for the boot time issue is not
yet in stable/12.)

The 12.0 schedule has been updated on the website at:

    https://www.freebsd.org/releases/12.0R/schedule.html

Thank you in advance for your patience.

Glen
On behalf of:	re_at_