defaults.retention-count and defaults.retention-age are not applied

In [backup definition documentation](https://backmon.dreitier.com/reference/backup-definition/overview) it is described that the following configuration is allowed:

```yaml

'./databases/{{client}}':
defaults:
retention-count: 10
retention-age: 7d
```

But that configuration does not have any effects on Prometheus metrics `backmon_backup_file_count_aim` and `backmon_backup_file_age_aim_seconds` (also probably `backmon_backup_latest_creation_aim_seconds`).
`retention-count` and `retention-age` can only be directly applied below the `files:` section.

Either change the documentation or change the behavior.

I've added a reference to *this* task in [backup definition documentation](https://backmon.dreitier.com/reference/backup-definition/overview). It must be remoed after we have decided how to deal with this issue.