[loki-distributed] Use ttl to replace deprecated validity

[lxwzy]

As docs says, the validity has been deprecated and we will got warning:

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

So I think ttl should be used here.

[CLAassistant]

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