roles/postgres: make the upgrade_pg.sh script more robust

Another thing that should probably be improved is the point at which the postgresql and postgresql-libs packages are upgraded. Currently that has to be done before running the upgrade_pg.sh script, but then postgresql.service may fail any time (but it probably won't cause database corruption).

This could be improved by adding the packages to IgnorePkg and upgrading them from the upgrade_pg.sh script along with postgresql-old-upgrade, which is after postgresql.service has been stopped. The condition at the start would have to be changed, since pacman -Q would give the old version. I'm using TO_VERSION=$(expac -S "%v" postgresql) in a personal script (which is based on yours), but expac is probably not installed on Arch servers at the moment. The subcommand should also skip the potential version from [testing], since that won't be installed due to [testing] Usage=Search Sync in pacman.conf.

CC @grazzolini