Make CortexIngesterReachingSeriesLimit warning less sensitive

As it turns out, during normal shuffle-sharding operation, the 70%
mark is often exceeded, but not by much. Therefore, this change sets
the new warning mark at 75%. It also increases the `for` duration to
15m as the expected reaction time for warning alerts is usually in the
order of hours, so we can as well wait a bit longer to see if the
problem is transient.

Signed-off-by: beorn7 <beorn@grafana.com>

cortex-mixin/alerts/alerts.libsonnet

@@ -255,9 +255,9 @@
                 (cortex_ingester_memory_series / ignoring(limit) cortex_ingester_instance_limits{limit="max_series"})
                 and ignoring (limit)
                 (cortex_ingester_instance_limits{limit="max_series"} > 0)
-            ) > 0.7
+            ) > 0.75
           |||,
-          'for': '5m',
+          'for': '15m',
           labels: {
             severity: 'warning',
           },