[Pgbuildfarm-members] add new target to buildfarm.conf

Now that release 8.1 has been tagged, you need to add a port for its
branch to your buildfarm.conf, if you are going to support the stable
branch.

I will be adding this entry to the distributed buildfarm.conf, and
cutting a new release within a day or two:

REL8_1_STABLE => 5683,

If you currently build the 8.0 branch, you might want to add this branch
to the set of branches you build for.

Conversely, if you now build for branches before 7.4 you might want to
drop an early branch from your set, althougn building for them is not
onerous, as long as you don't use the --force flag (which you should not
do anyway - it's there for debugging and cleanup, not for normal use)

cheers

andrew