Add slowlog thresholds to elasticsearch.yml template

[gherceg]

I noticed that slowlogs for elasticsearch haven't been written to since September 8th, which is when we upgraded from 2 to 5.

Looking closer at slowlogs documentation for 2 and 5, I'm not seeing an obvious difference in how slowlogs are configured.

However when talking about the various log level thresholds, it is the case that the docs say

By default, none are enabled (set to -1)
which to me implies that if none of those thresholds are set for any log level, we won't ever write slowlogs.

I don't see any references in commcare-cloud that would imply we are currently setting this value, and when querying a specific index's settings, I don't see any log related settings either. The only thing I can find that might relate to how slowlogs worked on v2 of ES are these lines in the logging.yml template, but I'm not sure if that is actually relevant.

@AmitPhulera I figure you might have a better idea of what changed here, but wanted to get something up so that we can resolve this.

Environments Affected

None

[gherceg]

Going to close this out as we will go about this another way which will be documented in https://dimagi.atlassian.net/browse/SAAS-15796