Use local time for Kibana logging

[ppf2]

In #8488, we addressed the timezone differences between Kibana's stdout and log file logging by using utc time for both.

This is an enhancement request to have Kibana change from utc to local time logging:

• This will make it easier for admins to correlate timestamps between Kibana and Elasticsearch (which defaults to local time)
• This makes logging consistent across products in the Elastic stack (the other products log using local time by default).

[alexfrancoeur]

Related to #8499

[epixa]

My preference in general is to have UTC as the default timezone for logs, but consistency is ideal here, so if Elasticsearch is committed to keeping local time as their default, then we can make that change for Kibana as well. To make this change safely without negatively impacting production installs that are relying on the current timezone behavior, we should tackle this in a couple of steps:

1. Make the timezone configurable Customize timezone in Kibana server log #8499
2. Deprecate the default UTC timezone in Kibana. This will need to be a docs change only to begin with, though we may want to add a startup warning if no specific timezone is configured as we approach the next major
3. Change the default to local in the next major

We're out of time for getting the timezone configurability into 5.x or 6.0, so the earliest we can kick this off is in 6.1.