Remove kbn-version from the response headers

[joshbressers]

Today Kibana has a header called kbn-version that returns the version of Kibana.

kbn-version: 5.6.0

This is often flagged by automated security scanners as disclosing too much information. While we do not consider this a security problem, and removing it doesn't increase security, it will cut down on the false positive rate from automated scanners.

The PR for this change can be found here
#20551

[kobelb]

We've gone ahead with this change, as the response header isn't currently used for anything.

[Erni]

A customer is asking when would be this change applied. They have a very strict security policy, and they need to know more or less the date when this change is gonna be available.

[kobelb]

@Erni this change will be in 6.4

[Erni]

Thank you very much @kobelb I will inform the customer right now.