postgresqlPackages: mark pg_auto_failover and pipelinedb broken for PG < 11

Marking packages broken isn't as fatal as throwing an exception. In my usecase,
I wanted to build all non-broken packages, that is:
```
nix-build -E 'with import ./.{}; with lib; flip filterAttrs postgresqlPackages (n: v: !v.meta.broken or false)'
```