use fifocache.duration instead of fifocache.interval in warn deprecated (#7609)
use fifocache.duration instead of fifocache.interval in warn deprecated (#7609)

**What this PR does / why we need it**:
I configured fifo cache with `validity` option (which is
[deprecated](https://github.com/grafana/loki/blob/b92f113cb0961b628b643f4f3f9fddc1c18ed921/docs/sources/configuration/_index.md?plain=1#L2004)
in v2.6.0, `fifocache.duration` as an flag) and loki print that:

```
level=warn ts=2022-07-08T17:01:30.134251284Z caller=fifo_cache.go:125 msg="running with DEPRECATED flag fifocache.interval, use fifocache.ttl instead" cache=frontend.fifocache
```

However, there is no flag `fifocache.interval`.
As #5148 changed, the deprecated flag should be `fifocache.duration`,
not `fifocache.interval`.
grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki