fix: use LESS_THAN_THRESHOLD for addMinUsageCountAlarm (#318)

`LESS_THAN_LOWER_THRESHOLD` is for:

```
* Specified statistic is lower than the anomaly model band.
* Used only for alarms based on anomaly detection models
```

...which is not what it was being used for.

---

_By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license_

Author: Eugene Cheung

lib/common/monitoring/alarms/UsageAlarmFactory.ts

@@ -47,7 +47,7 @@ export class UsageAlarmFactory {
         props.treatMissingDataOverride ?? TreatMissingData.MISSING,
       comparisonOperator:
         props.comparisonOperatorOverride ??
-        ComparisonOperator.LESS_THAN_LOWER_THRESHOLD,
+        ComparisonOperator.LESS_THAN_THRESHOLD,
       ...props,
       disambiguator,
       threshold: props.minCount,