safe_wal_size not available on pg12

[michael-todorovic]

What did you do?
I'm running postgres_exporter in multi-target mode and target a pg 12.17 cluster

What did you expect to see?
No issues

What did you see instead? Under which circumstances?
I got the following error mesage:

{"time":"2024-11-27T16:42:03.793572559Z","level":"ERROR","source":"collector.go:201","msg":"collector failed","target":"<server>:5431/db","name":"replication_slot","duration_seconds":0.028229965,"err":"pq: column \"safe_wal_size\" does not exist"}

This column exists starting pg13 so the exporter is likely missing a version check

Environment

• postgres_exporter version:

postgres_exporter, version 0.16.0 (branch: HEAD, revision: a324fe37bca5193a293118b940b3df7ab3a8505c)
  build user:       root@buildkitsandbox
  build date:       20241127-14:34:19
  go version:       go1.23.0
  platform:         linux/amd64
  tags:             unknown

• postgres_exporter flags:

    --config.file=/app/postgres_exporter.yml
    --log.format=json
    --collector.postmaster
    --collector.index
    --collector.stat_user_indexes
    --collector.stat_checkpointer

• PostgreSQL version:

  12.17

I'll try to provide a PR for that