pg_upgrade analyze script

For a long time now, pg_upgrade drops a script (analyze_new_cluster.sh)
that just calls vacuumdb to run the analyze in stages. This script made a
lot of sense back when it actually implemented the stages, but these days
since it's just calling a single command, I think it's just unnecessary
complication.

I suggest we drop it and just replace it with instructions to call vacuumdb
directly.

Attached patch does this. It also removes the support in the instructions
that talk about pre-8.4 databases, which I believe is dead code per
https://postgr.es/m/CABUevEx-D0PNVe00tkeQRGennZQwDtBJn=
493MJt-x6sppbUxA@mail.gmail.com.

-- 
 Magnus Hagander
 Me: https://www.hagander.net/ <http://www.hagander.net/>
 Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>